At this time of year we see a spike in UPVC door and window related issues. Increased heat can cause UPVC to become more flexible. Bi-colour doors that are dark outside and white inside are more likely to warp if they get the sun on them. As the ground dries out buildings can shift leading to door misalignment. All of these can cause a door to be difficult/impossible to lock/unlock.
Spraying WD40 or similar isn’t going to realign your doors, nor stop the house settling. If you ignore a door that is difficult to lock don’t be surprised if it breaks. It is always cheaper to correctly align a door than to open a jammed shut door and replace the multipoint.