Had a problem with printing as well, was fine in the beginning.
I thought a bow-den type feed would be better and solve most of my problems with feed rates so I stripped the extruder feed unit to see how I could modify it. On checking the parts in the extruder feed, [ drive unit feed unit (( the bolt with splines on it driven by the gears and motor )) I found them to be clogged up with somewhat ground up PLA dust which I cleaned up including the framework parts. Then decided to check the BEARING TYPE: 608. Found that these were also jamming slightly on the drive shaft and the tension adjustment bearing. I found this bearing to be completely clogged up with dust and not able to turn at all, cleaned up all the bearings. PITY THEY WERE NOT THE TYPES THAT HAVE DUST COVERS ON. SURELY velleman COULD HAVE PROVIDED BEARINGS WITH DUST COVERS WHICH ARE 608ZZ TYPES, so I have ordered 3 new bearings with rubber dust covers from eBay.
This caused the extruder to give me low feed rates [ mostly lack of feed at times ] causing many a failed printed project to be scrapped. I had spent many hours trying to find this fault, even replaced the extruder with a V6 extruder unit with cooling fan.
BTW The extruder feed unit can be separated from the nozzle by fitting it with bow-den type tubing and mounting the drive unit on the upright just above the CPU Board and remaking the brackets for the extruder unit [if you using a extruder head similar to the V6 unit] being careful to only use the 6mm type fittings and carefully tap out the entrance and exit points for the filament, also turn the spool mounting post to allow the filament to feed directly to the extruder drive unit. This also makes the feeder bar lighter and easier for the z axis motor to rotate easier.
velleman should have provided the extruder with 608 ZZ bearings I’m not saying this is your problem but might help