I agree, the triangle nets i've used were actually ridiculously difficult to capture a shrimp in. You'd think it would be easier to corner a shrimp, but they shrimp just jumps out the top of the net.
I never corner shrimp though, I always allow them to jump back into the net by touching them gently on their tail with the net rim. Takes a while to master this method, but works every time for me now.