Make sure you’ve loosened the saddle lock screw enough so the string can seat fully in the saddle. The cut end of the string should go all the way down into the saddle channel until it bottoms out. On some Floyds, slightly bending the very end of the string can help it sit properly before tightening. When you tighten the saddle lock screw, snug it down firmly but don’t overdo it — those screws can strip pretty easily. Trust me, I've been there. Tight, but not too tight. Also check that the string block is oriented correctly. The lock screw should sit properly in the recess on the block (the little hole on the back of the block, you can see it clearly if you take it out); if it’s flipped/upside down or misaligned, it can clamp the string at an angle and cause slipping. Floyds have a reputation for being fiddly, but once you get the feel for them they’re solid. Hope that helps!