My name's Kel.
Unless your home's on poor soil -- probably not the house shifting.
The door the storm's attached to would have settled a bit and the storm itself may have sagged with use.
First -- take out one of the screws that hold the storm frame to the door trim. Get a slightly longer set of screws. Remove the old screws and use the longer ones to pull the frame enough to allow the door to close smoothly.
If that doesn't work -- how skilled are you with a circular saw?
Remove the door and cut wedge shaped shims to hold the storm frame level. You'll have to be able to rip a shim upwards of 36" long that tapers from 0 to how ever much the door's out of level -- probably at least a 1/4" of an inch.
You'll have to cut the storm frame to fit in the slightly smaller opening. You'll have to adjust the sweep at the bottom of the storm, so it fits correctly.
If you're not up to these operations then it may be time to hire someone.
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