Krill (prawn/shrimp) baits are normally used when the fish are active. So you should be looking for large movement in the float, and then striking when the float almost dissapears into the water.
When the fish are active the float will sometimes completely dissapear beneath the water!

When using paste baits it is best to let the fish take the bait completely into their mouth before striking and setting the hook. This will lower the chances of a bad or weak hook up.
Try waiting until the float has dissapeared beneath the water, take a breath and then strike!