if you suddenly lost lock, not sure, but I don't think that the 501X has a built in servo saver, so won't be that. You would have to rule out a fouling ball stud coming loose, one-way cup coming out.
Best thing is to disconnect the track rods at the wheel end and check for full steering range, if you don't get it, check your transmitter for full steering, then remove the steering link from the servo and check it throws fully. If it does there is a restriction in the steering rack, if it is still short, tranny adjustment or bust servo.
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