I did have a similar problem some time ago - it seems that the plastic 'dowel' which holds the end of the blind into the the slot in the roller tube can slide outwards and make the blind catch on the left or right sides - often accompanied by a 'clicking' sound as you pull the blinds up or down !
If this is the case, you need to remove the side runners from the window and trim the excess dowel off the ends where blind slots into the roller tube,(it is like thick nylon wire), no point in trying to push it back in 'cause it'll only happen again !
Hope this helps, sorry if it's not made very clear, not easy to explain without pictures !
If however this is not the problem you'll need to try re-tensioning the roller spring.