The wall fridges are unique and cool to have in any kitchen!  They have quite a few unusual aspects to them.  Because they are more or less a built-in appliance, people will go to great lengths to get them repaired since replacement isn't an option and the kitchen isn't complete without them. Due to the fact that there are few people who proficiently still work on them; many have had horribly botched repair attempts done out of desperation. 

Because of that, you have to be extremely careful to do thorough troubleshooting and leave no stone unturned. As John said, the early ones often had wear-out problems with the compressor. Many have newer compressors installed from "back in the day." Many of the replacements have, it seems, begun to lose performance as the decades passed. 

The most recent one I worked on had a fully blocked capillary tube due to a bad filter-drier. It caused the compressor to run but not cool; and then to act as if it were seized up. In reality the compressor was still perfect, but the blockage "outwardly" emulated a failure of the compressor, when a technician did not take the step of using his gauges to make a proper diagnosis.

The design of these fridges is such that the refrigerant first cools most of the freezer, then cools the fridge compartment, and then returns to the freezer where the suction accumulator is located. If the refrigerant charge is reduced, it will first stop cooling the refrigerator cabinet evaporator coils, before the freezer loses cooling.  

The refrigerator part contains the one and only thermostat. Once the charge is low, the fridge will run all the time, resulting in an abnormally low freezer temperature and poor fridge cooling.

There are copper-to-aluminum joints inside the cabinet where the tubing connects, as well as braze joints on the condensing unit. It would be prudent, in my opinion, to start with visual checks and be sure there are no obvious oil leaks on any lines and to be sure the fan is running and the coils are clean.  Then from there, take the upper right cover off, inside the refrigerator cabinet and inspect or "sniff" the copper-to-aluminum joints for refrigerant leaks. 

The bottom line is, leaks are fixable, but some take a lot of time and effort to find and fix. It is very likely you will need to have a specialist make the repairs unless it's an obvious leak that any qualified technician can repair.

Hope this is helpful, and good luck getting it going!
Sincerely,
David

Robert; I can't really comment on the commercial fridges because I just have no experience. 

As for connecting gauges, you do lose some refrigerant into the gauges; however it is not as big of an issue as you would think.

If you have a blockage in the system it won't matter that some refrigerant is trapped in the gauge hoses, because you are looking for a change in readings as the compressor is started. You'll see the two pressures start about equal. When the compressor is started, the low will rapidly drop, and the high will rapidly rise.  Then as the low pressure stabilizes at a vacuum or very low pressure, the high side will remain the same, or possibly taper down as the trapped liquid cools off.  Once you stop the compressor, if it is a blockage the gauges will not equalize back to equal high and low side pressures. The low will stay very low, and the high will stay much higher. 

If the system is working correctly, it will only take a matter of minutes before the two sides are equal in pressure.

If the system is working properly, you can return the majority of the refrigerant from the gauge hoses back to the system. To do this, close off the service port on the high side of the machine. Either close the line puncture valve (if that is what you have used) or close off the service hose valves, if your gauge set has them. Once that is done, open both the high and low pressure valves on the gauge manifold. Do this gradually, so as to allow the refrigerant to draw back into the low-side of the system. This will ensure only a small amount of vapor remains in the hoses. There is some loss, but it will be minimized. 

Hope this helps to explain it. :) 

Robert; glad you found that informative!   One thing to note is that the light can add a "significant" amount of heat load if it stays on all the time. It shouldn't make the fridge totally not cool but it could cause very long run times and short off-times, or constant running.  Also if the door gasket is in poor condition, enough, that the light shines out that will hurt performance badly.

After it's been running, look up into the top of the cabinet at the refrigerator evaporator coil.  It should have a layer of ice on it from one end to the other, while the unit is running. The thin layer will form while it's running, and then melt away after it shuts off. If you see that the ice is only forming at one end of the refrigerator coil, that's a pretty sure sign of low charge. 

If the ice is forming all across the coil, you  may want to remove the lightbulb and see how that improves the performance.

Sincerely,

David