Hi Kristy!

A couple of things that come to mind that may help...

Is there much fluff in the velcro grip tabs? It will hinder their stickiness if there is.
Is there much pilling on the loop tabs? Same thing will occur as fluff bunged grip tabs. Very carefully trim off the pilling.
How long have you had them for? They do have a warranty so perhaps email the seller to discuss JIC there is faulty velcro.

Good luck I have 4 BumG's the oldest about a year old now and the velcro still grips nice and strong but I do have to clean it up from time to time.

HTH