I've found that PBT space bars frequently have this trouble, which is why ABS space bars are more prevalent, even if the rest of the keys on a keyboard are PBT. I can usually work it out though by evenly heating the space bar with a hair dryer (wearing gloves to protect my hands), then carefully bending. Test by seeing of there is any wobble on a flat surface. Too much bending in one spot can cause a crease mark in the pastic, so be careful. The key is even heating and bending a whole section of the space bar, not just a single area of it.