Some tefillin have a tendency to bend on the bottom. This problem is more pronounced in the wintertime. Although they are not posul if they are only bent a little bit, they should still be fixed. This is because when they bend on the bottom they tend to pull apart the four compartments of the shel rosh. When this happens they become not square. You will nottice this problem more in a shel rosh than in a shel yad. If your tefillin are gassos (made out of the thick leather of the neck of a cow) this can be fixed with a heat press. Sometimes soferim who don’t have the equipment to do this with heat will put them in a vise and skip the heat part of the process. This rarely works. If the heat is used, and the battim are sealed up properly afterwards, it should be a good long time before you see a recurrence of this problem. By the way, using too much heat will melt your battim. Not fun.