There are a number of factors that can affect the level and strength of suction. The most common reason is the seal between the base of the gaiter and the pelvis.
To achieve the best possible suction ensure you trim your pubic hair as short as possible (or shave completely) and for added benefit apply a little Vaseline around the base of the gaiter.
For additional comfort and suction, some users also find the use of a universal pump sleeve of benefit which can be purchased from the likes of Ebay and a variety of popular online adult product stores.
The positioning of the base of the gaiter is also very important so you may find that rotating the base of the pump into a different position can provide a more snug fit to your body and hence allow you to achieve a better level suction.
A more drastic approach involves using a razor to very carefully trim the base of the gaiter so it better adheres to the contours of your body however please note that doing this will automatically void your warranty for the respective gaiter(s).