The top slide is carried on the V (sideways) on each side. The lower side of each V is horizontal. The gib screws press on the gib, effectively pulling the slide over to their side and tightening the slide into the opposite V. At the same time the gib presses on the top of its own V forcing the base of the slide into contact with the lower side of the V. As long as this is what you have, it should always be possible to 'overtighten' the gib screws and lock the top slide down onto the horizontal surfaces on both sides.
So, it seems that you may have your gib screws mis-assembled, not locating in their cut-outs (or dimples). Or possibly a home-made gib strip which is not big enough?
Alternatively, it may be that the up-&-down movement you are feeling is not the top slide itself but below it. Perhaps the saddle on the bed, or the cross slide?
Hope this helps
Regards, Tim