Hi, I'm not certain what type of wiper you have, but if it is always parking in the same spot, it is most likely due to the crank slipping on the shaft. Most wipers have a splined shaft, often tapered, that the crank fits on, some have one or two flats on the shaft instead. The crank fits over the splines and is held on by a nut. If you turned wipers on after a long period of non use, or if you window was gunked up, sometimes the spindle will turn inside the crank. Once the wipers have picked up and sweep properly, the only sign of this is self parking in the wrong spot. It is possibly an electrical fault, but not that likely. Best way to check this is to remove crank from wiper, inspect the shaft for damage. Then turn key on, operate the wipers one sweep to self park, TURN KEY OFF, then lower blades to park position on screen, reattach crank to spindle. It is essential to turn key off as if you swing on the spindle nut with key on, the wiper park mechanism is still live, and if the internal gear wheel turns even a mm or so it can set the wiper in motion for one complete turn. If spanner is still attached it can create problems, maybe blow fuse.
If fingers are in there anywhere, it can get very messy, and will no doubt cause loud obsceneties to erupt from within you.
It's a good practise to flick both blades off the screen once every couple of weeks, more so in summer, as they tend to stick after a while, damaging wiper motors on start up. Sea spray and sappy stuff from some leafy trees cause a lot of problems.
Hope this helps.