If a roller-blind will not wind itself up fully or springs back too forcefully, the probable cause is that the ratchet inside the roller needs to be tightened or loosened respectively.
If the blind is too loose, pull it down to its full extent, lift it off the support brackets and roll it up by hand. Replace the blind in position and slowly pull it downwards again. If it is still too loose. Pull the blind until it is half-extended. Then remove the roller from the brackets again and repeat the hand-winding process. If the roller-blind springs back into position too forcefully. The ratchet should be loosened. Make sure the blind is fully wound, remove it. And unwind it half-way. Replace the unwound blind and test the tension. If it is still too great, repeat the process. If either method fails, fit a new ratchet.