Hello Joanna,
It would appear you have the seats as per the Video that Neil so helpfully included.
I have included another here, which may be also helpful.
So we need to establish whether your seats(s) is broken, or just that you have not yet figured out how to operate it properly.
I have a seven seater, with different seats, but do just so happen to have a single swivelling seat in my garage.
Both these videos show that the slider movement is released (and relocked) by the lever at the front of the seat.
From your description you appear to have full functionality except for the relocking function. Am I right ?
The only help I can offer is to suggest some small jiggling of the seat position and waggling of the lever in the hope that the seats will find a locking position.
It is possible that that both seat mechanisms are broken; but that, to me, seems very unlikely as they are independent of each other.
It could be the rails, but as both seats are mounted on independent rails; that seems very unlikely also.
If all else fails, the seats can be removed for inspection quite easily, but they are very heavy.
Just four coach bolts for each.
I'll check my seat in the garage, and my copy of the JPNZ Owners Manual (which you should get if your don't have one already).