Sometimes, The E-Pick and Airpick have a level of vaccum upper than the maximum level of Vaccum set.
The Epick is just a pump that removes the air. It can be activated and deactivated.
For exemple, if you ask the gripper to have a level of vacuum between 10% to 20%, the gripper will start the pump until he has 20%, then he will stop.
If you have a nonporous material and you try to catch an object far away from the vacuum like that : 
The pump will be activated for a long time to have enough power to catch the object. So then when the object is catch the level of vacuum is more important than 20%. In that situation we have 40% :
We cannot have a level of vacuum lower because the gripper cannot put more air inside. So the gripper can just stop and wait that the air come in the valve. In the case of nonporous material, the level will stay stable but in a case of a porous material, the level of vacuum will be lower and lower until the minimum level of vacuum. So then the pump will be activated again.
To solve this problem, you can try to touch the suction cup to the object you want to take, like that : 
Then you have a level of vacuum lower :
