One boob?

Ti

Need some help. I am 5.5 months into a mix of breastfeeding and pumping. I have a medela pump and usually pump both sides at the same time. I went to pump this morning and one one boob was getting enough suction so I checked the tube, nothing blocking the flange, the pieces were snug, etc... so I swapped the tubing between boobs to see if that helped and no go. The one boob still didnt get going. Thoughts? Is it the membrane?