You might want to read my posts on this - it's a real can of worms.
The little gears inside the motors wear. Sure, there are replacements - but they often don't mesh properly, and it will just jam.
Even if they do, the motor casing (made of putty) will wear - the gears bear directly in holes drilled in the casing, no bush, and the tiniest bit of wear will make them judder and jam.
I spent weeks trying to fix mine, trying pretty much everything you can buy. Much of the stuff on eBay is absolute {naughty word} and you'll drive yourself mad wondering why it sticks etc when you'd put everythgin togetehr so carefully.
I ended up buying a new one myself, in the end, only to have it fail after a year
Hope you have better luck if you decide to have a fiddle
