Did you paint or varnish the door yourself? Door panels need to be able to "float" freely so they can expand and contract with humidity independently of door parts with grain oriented differently. When incautiously finishing the door with anything but stain, the finish may act as an adhesive, locking the panels in place and causing them to crack when they shrink during a dry season.
Been there, done that, despite knowing better.
Assuming that is the problem, fixing it is tough.
IF you can get the panel free again (careful application of razor knives and very thin metal shims), you may be able to hot-glue on some clamping blocks to pull the crack closed after applying a very carefully limited amount of glue (just enough to hold it together, not enough to recreate the problem). This will preserve the strength of the door while restoring its appearance.
If you are willing to weaken the door and damage its appearance somewhat, you could try cutting away just enough of the door to free the panel pieces, glue them back into a single panel, and then try to nail/glue in molding to replace what you cut away and hold the panel in place, then try to color these new pieces too much the rest of the door. While, again, trying not to recreate the stuck - panel problem. I can't see enough of this door to have an opinion on how hard it would be, but it does risk making a bad situation worse. If you try it, I highly recommend working on the inner surface of the door, so your mistakes are hidden from the public.
Then there's the ugly solution: just inject a flexible caulk into the crack, which will plug the gap, though it won't be pretty. If the door is painted, use a paintable caulk and paint the filled crack
to match the door. If the door is stained, all you can do is try to find a caulking compound whose color matches the door well enough not to be noticed.
For the door that I botched, I tried the second and third fixes; see photo. The stain on the replacement trim around the top panel could certainly match more closely, as could the shape of the trim (ogee rather quarter-round) but this was close enough that it didn't bother me too much. The bottom panel was fixed by injecting dark brown caulk; if you look carefully you may be able to find the break line.
If this was not a self inflicted wound - if somebody else finished your door - complain to them. This really shouldn't happen if the finishing job has been done properly.
I bought this front door and after a few weeks it started to separate. Got through winter and it continued to get worse. I am hoping to find a solution to repair the door since the seller left me high and dry. Now that it has been a year, the sap should stop leaking out. I'll scrape that and sand those areas. For the separated planks, I was going to use a black wood filler - apply it from the back of the door, sand the back, put a piece of wood over the back on the inside and then paint the whole door. I am not sure if this is the best route or how it would look on the front of the door. Any suggestions?
