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1 may have gone out sometime ago, but with all units working, it didn't effect the temp of the box. Could also be a restricted LLFD.

Can try putting the TXV bulb in hot water and see if pressure rises. But if the LLD is restricted, it will still act like a bulb lost its charge.

I take it for granite that the sight glass is clear.

I'd probably recover the entire charge, replace both TXVs and the LLFD, vacuum and recharge the 34 pounds that was written down. Restart that unit and make sure sight glass is clear and check SH.
 

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With little refrigerant flowing through the evap, there is not much heat to get rid of, so head stays low.

A LLFD can have a large pressure drop and not have any temp difference across it.

Get out a PT chart for R22. Lets say its 95°F outside. And the head pressure at the service valve is 260PSIG. And you have 12° of SC. If the LLFD has a PD of 34 PSIG across it. The saturated temp of the R22 is 110°F, so you won't see a temp drop until the restriction has a 50 PSIG or greater pressure drop across it.
 
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