I have had this issue and it is fairly simple to fix.
You need to remove the wiper arms from the splines (with care!).
Then remove the plastic panel (a few screws) covering the area under the windscreen (it's in three parts). Disconnect the washer tubes as you lift this away from the car.
This exposes the wiper assembly. At this point you can remove all the other detritus like leaves and pine needles etc.
You can either remove the entire assembly and work on the bench, or do the job in-car.
You need to remove the white plastic cover over the 'gearbox' part of the wiper motor. Do this with care as there is a plastic clip as well as some screws (from memory).
Once the cover is removed, you will see the copper tracks on a toothed disc, and a load of grease.
If you want to remove the grease or move it around, do so, but the fix is to gently bend the three (or four, can't remember) copper spring contacts so they make a better connection to the tracks on the toothed disc. Obviously take care so as to not break the contacts.
Then put the assembly back together.
When replacing the plastic panels (I don't know what these are called really) below the windscreen, you need to part-assemble some plastic parts that fit under the windscreen lower edge. This is probably the most complicated part of the operation. You'll figure it out I'm sure.
You may want to consider re-painting the panel before re-fitting as it weathers with age. I did and it looks like new again. Satin black.
When you re-attach the wiper arms, ensure the teeth mesh on the splines. If there are no teeth left, it's time for a new arm or spline. I think this is a routine maintenance item.
Good luck, and I hope this helps you achieve a smooth wipe!
Dave
