cheers our 1/4 slides in real easy even with new to our beetle used frame, didnt think about the adjustment on 1/4 top screw. Will measure original and replacement 1/4s.If the top ends of the channels haven't been crushed (making one or both of them too tight), it could be that the channels are not perfectly parallel, (too close to each other at the top). The rear channel only really has one position, so the 1/4 light is suspect.
There is a tag at the top of the 1/4 light frame which gets screwed to the door, the tag has an oval hole to allow forward/rearward adjustment, (not a lot, but should be enough).
However, the tag often gets bent when removing and re-fitting the 1/4 light, leading to misalignment, and as you mention that this is not the original 1/4 light, I would check there first.
Often the 1/4 light frame is difficult to fit into the door properly, as the rubber around the front edge can make it difficult to 'seat' the frame into the door, (especially if the rubber has become hard due to age). I've even had to use clamps to get a 1/4 light to go into the right place in the door. Lubrication can help, but it will always be a bit of a wrestling match.
