Hi Dave
sorry to hear of your problem.
A memory just came to me, about 20 years ago I built a porch for a customer, When it came to the door I gave the chap a catalogue from a firm that makes hardwood doors with Mortice and Tenon construction, the price for the door he wanted was in the region of £180.00 then, he said he would not pay that much and said that he would find a cheaper one, well he got one and when I seen it I told him it was not good enough for an outside door, he said "just hang it it will do me" I gave it two coats of outdoor yacht varnish, let it dry and hung it. I went on holiday a few days later for 2 weeks, a week or so after I got back he called me and asked me to come and fix the door as it was sticking, I took one look at and told that is what happens to cheap doors, he argued and to shut him up , I put a pair of sash cramps on the door top & bottom tightened them up a midges, released the cramps and the door closed as sweet as anything.
The moral is for a good outside door that will give no problems for 20 or more years have one made with M&T joinery,
The ones you buy at the big box stores are invariably made with dowel construction and they tend to come apart in any type of weather.
Maybe this is what you have. Check if have any gaps between the stiles and the rails, even minute gaps will cause the door to stick, also has the door had the leading edge champered.