Vertical (forward and back) blade play is usually and issue where the tang of the blade and the end of the lock bar do not contact each other tightly. This can be a machining error (like on a cheap kershaw I had where the tang was ground too much) or the end of the lock bar has been dented in (which can happen when someone spine whacks the knife, or batons with a folding knife locked open). Or just poor tolerances in general and being a CRKT, they don't usually have the best fit and finish or tolerances, although my only example (a Caligo) is pretty nice and worth the $30 I paid. they're $40 now and I wouldnt recommend them for that but I digress...
How far has the lockbar traveled on the blade tang? if it has reached, or almost reached, the opposite scale its pretty much worn out. if it's not, you could try bending the lockbar over a bit to see if you can get it to go over further and lock up better. or you can peen the lockbar (use a punch and hammer to squash the metal a little and get earlier and tighter lockup.
Another possibility is that the pivot is out of tolerance (undersized) and is allowing the blade to move around. Sometimes vertical play on Benchmades is attributed to this, and it can be a factor, but if the other contact points are solid (stop pin and lock bar or axis bar) it is usually not a big factor. I have plenty of knives where, when unlocked, you can pinch the blade and move it forward and back and feel it move. Once locked up there is no movement however, so a slightly undersized pivot may contribute but probably wont be the biggest factor. If you suspect it is, I think you can get aftermarket hardware for it, or even contact CRKT and maybe they can send you some! I hear they have really good service.
Hope that helps!
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