You probably already know this, but to get maximum suction for vacuuming:
1. Shut off the main drain valve and open the skimmer valve all the way.
2. Use flaps or rubber plugs to close off the other skimmers that you are not using for vacuuming.
If your pump flow rate is normal for your size pool, you should have no problem vacuuming. If you are, you may need to repair or replace your vacuum head or you may have a plugged vacuum hose or lint filter.