Before you spend money on a new slide, consider checking this first:
What do you mean when you say the mags don't have followers? In almost all GBBs the follower has the little bar that makes the slide-lock function work... so that's confusing.
Assuming you really do have followers, so the mags ought to work, take the bottom off the mags, remove the springs and followers, and see what condition they're in. I had one (also in a KSC but a glock) where the spring was getting weak, worn, and neither it nor the follower were sliding smoothly.
I replaced the spring and follower with new, and the slide-lock worked perfectly after that.
Hope it helps,