One of the most important things when it comes to pumping is getting the right size breast shields. The standard flanges that come with your pump may not be the right size for you.
If a breast shield is too small, the nipple can't move freely in the flange tunnel. If it's too large, too much of the areola is pulled in to the flange tunnel.
If you think you may not have the right size flange, you can try measuring your nipple with a ruler, get sized virtually, or just try a bunch of different sizes to see what works best.