Bought 14mm rear hub adapters and new wheels but the nylock wheel nut keeps coming loose and falling off. Can i put thread locker on them to keep them on?

Ideally nylocks are used only once since they lose their locking ability after a few uses. Not practical in rc cars, but that's what my engineering side knows. Just use purple or blue on a full metal nut with starlock washer or built into nut. Loctite will eat the nylon.

Bradley is correct but I'd just use the nylocks and, IMO, blue loctite/thread locker. The nylon provides zero mechanical strength so it doesn't really matter if it reacts with the thread locker. In my experience Nylocks also "handle" much better when using a standard lug wrench or nut driver. Axles are usually designed as shoulder bolts & may require specific hardware. The nuts tighten up against a shoulder (usually) & capture the inner race of the bearing while not being large enough to contact the shields/seals. There are a TON of options for a specific thread & some will have an outside diameter that is too large & may contact the shields/seals of the bearing. Be careful with loctite as it can have the same effect if too much is used. I put a drop on the back of the nut & then just wipe any excess with a paper towel. Also be sure to give it some time to set before use. I keep new/gently used Nylocks around for when I can't wait on the loctite like during a race day.