I have had quite a few fail with the plastic inserts, not on my machine but on a different job which I use them for, switched to the slightly more expensive metal insert ones and they have been fine.

Well your luck may have changed if its a 25mm one as i have box full of the metal insert ballnuts and you can have one for a good price.

From experience though the 2 types do not fit on the same ball screw so don't get too excited just yet...