The first thing I will ask, what size tube has the pump got. If it is the 50mm (2")tube, they will get you worms, but very hard work if the the sand is compacted with clay and mud.
Ideal size is the 28mm which makes things a lot easier on the back and arms.
Different people use different methods like, placing the pump directly over the wormcast, drawing the pump handle straight upwards, the shorter 75cm. Pumps are best suited to this method.
The method I use is, place the pump about 3 inches from the cast, at an angle, pushing the pump, at the same time pulling the handle, aiming for about 6-8 inches directly bellow the cast, fully drawing the pump handle up, this style works well with the 85cm pump, and is much easier on the arm and back.
When you first use the pump make sure it is lubricated with a small drop of washing up liquid, and the end wingnut is tightend enough to give a good suction against the palm of your hand.
Where abouts are you!