Can you take apart rear derailleurs, and how do you put them back together again?
In my experience the metal caps are mated to the wheels, and while the jockey wheels look the same they are not. Every time I have found the wheels not spinning it has been a simple case of having the wheels in the wrong position or being mated with the wrong bearing cover plates. If you have both wheels not spinning then I think you probably have bearing plates swapped; if one turns and one does not then you might have the plates on correctly but the wheels swapped causing the thicker wheel to be clamped tight while the thinner wheel is barely touching the cage.

I have never used loctite when reassembling bike parts, just care in assembly and a checkup tensioning after a couple of rides. In fact I often dab a little grease on threads to avoid rust binding.

It is probably worth buying jockey wheel replacements if you are servicing a 10-year-old derailleur. If it's a Shimano or something else relatively common chances are you can buy original replacements or even fancy anodised aftermarket parts.