As Andrew says i would put a couple more dimples in the gib to mate up with the other two adjustment screws. I find that these gibs tend to be of questionable quality at the best of times from the manufacturers. Chinese QC is one step above a chocolate fire guard to my mind . I had a similar problem with my WM 250V-F, bought back in 2012, I ended up making a new gib strip, from brass as I had no suitable ground flat stock, it's been in since then without any problems..
OEM gib strip as supplied.. nothing but a strip of scrap steel!

New brass gib fitted… as can be seen the top slide corner undercut is way to wide..

Another mod added.. top slide gib lock..

George.