Bowden extruder on cherry pi delta 111 keeps blocking

I've been having this problem where I have to tighten the filament tension screws quite tight to get it to grip when extruding. If I turn The gear by hand, it extrudes fine but if I keep turning there's eventually some back pressure and when I let go of the extruder, the gear turns backwards because of the back pressure. I try to extrude again the nozzle is blocked. The worst part is that I've replaced my j head, thermistor, hobbed bolt and I know it's good quality filament as I've used this filament for ages. I also have a fan on the side of the plastic jhead. I'm trying to extrude 1.75mm pla.
Thanks for any help I get.
Cheers
Ben

Have you tried increasing the hotend temperature? I had the same problem, tried replacing the bowden tube and all sorts, turned out I just needed to increase the heat a little. You should be able to push the filament through manually with just a finger and thumb, keep increasing the temperature until you can(within reason of course).