This is a really common problem, and one that is annoying yet relatively easy to fix. If your roller blind has been opening or closing unevenly then it can cause problems with the mechanism, over time this can cause it to jam. When this happens then you might find that the blind material is jagged or has loose thread on one side. If so then then it is worth fixing this before looking at the mechanism. This is because if left as is then it can cause further jamming when the mechanism is fixed. All you need to do is trim off any loose threads and ensure the edge of the fabric is smooth. You may need to trim a few millimetres using scissors if required.
Next you will want to remove the blind from the window, by un-clipping it from mounting brackets. Once down you will want to unroll the blind and then manually (not using the blind cord) roll the blind back up, ensuring you roll tightly and evenly. The roll needs to be neat, just how the blind looked when you bought it. From there place the blind back in the bracket and again by hand pull the blind closed half way.
This will restore some tension to the springs within the blind, which will make the blind cord work again. You then need to replace this step several times. Take the blind down, open it fully, roll it tightly back up, place within the mounting brackets and then pull the blind open. After three times you will want to pull the cord the normal way to open or close the blind. You should see a marked improvement in how the blind operates. This simple technique is enough to restore the tension levels of the spring within the mechanism, allowing the opening mechanism to work as designed.
If this doesn't resolve the issue then you will need to remove the blind again and adjust a tension pin on the side of the headrail mechanism. Take the blind down and unroll the blind so it is in the half open position. Do this by hand rather than by using the cord. There should be a tension pin on the headrail, using pliers twist this pin clockwise until you can feel tension. Then release the pliers and let the pin rotate anticlockwise a little to a resting position. Lifting the blind back into the bracket should now be able to open and close the blind as required.