i hate them too! Mine are all nicely greased but the drivers side on seems to struggle in the middle for some reason.
I would do the following:
Pop off the white base rollers, clean and grease. Use a light grease
Clean the centre channel on which the main slider slides. I would use either a heavy oil or again some light grease here
Finally, make sure the two side channels are clear - use a piece of cloth to remove any loose debris. Clean and spray with a silicone spray.
If its still the same, you'll prob need to take the motor mech out, clean it and grease it up again. Not fun.
But hopefully the first few steps should sort it
Hasan