Can knees lock up after total knee replacement?

Yes, but not often: Knee replacements can feel like "seizing" or "locking up" on rare occasions and they are usually not due to the device. The most likely phenomena you are experiencing is either "crepitus" or "clunk" which is when scar tissue surrounding the front of the knee gets trapped within the device creating a sensation of locking up. When severe enough, it can be alleviated by removing that scar tissue.