There's no simple answer.
There could be many reasons for this and to be able to tell the difference comes down to experience. Sometimes the fish can 'shy off' meaning that they feel a bit unsure if it's safe or not and so tend to act skittishly around the bait, with regular feeding and minimal disturbance you can get them to settle down. Fishing later in the day can put them off if there's a lot of barge / lock activity as your bait can be washed all over the place.Sometimes the fish will come up in the water and snatch at you bait.
Now a list of (sometimes contrary) possibilities:
The hook's too big
The hook's too small
The bait's too big
The bait's too small
The bait isn't hooked right
You're fishing too deep
You're fishing too shallow
Shotting pattern is wrong
Too much disturbance or too noisy around you
Too hot
Can see the shadow of your pole waving overhead
Line's too thick
Float's too big
Shot's too big and obvious
It can depend on what is actually biting, smaller fish can play havoc as they tug on the bait but don't take the hook in, if you are using grubs (maggots), a sign of this is when the bait comes back chewed or flattened (a good sign of roach), small fish tend to struggle with pellets either hair rigged as they don't take the hook in as well as the bait or the pellet banded to the shank can prove too much of a mouthful.
Sometimes you just have to give them time to take the bait in or put a smaller or bigger hook on, maybe hook the bait in a different way. You could try a different bait, if I think it's smaller fish I'll put a tiny piece of bread on a 20s or 22s hook and see what happens. You can often feel small fish wriggling like blazes when you lift into the bite, this is a give away, or if you feel a weight and then nothing, you have something to go on.
Changing depth can help sometimes, the bigger fish tend to stay lower in the water to sweep up what the smaller fish let through. You could change your shotting pattern to put the bulk of the weight lower in the water. Try changing your feeding pattern, either feed first then lower over the top or lower your float in and feed over that.
There may be pike, perch or other predators in you swim.
There could well be many other reasons, and it will change, just keep thinking about what's going on under the water and make changes (one at a time) until you can figure it out, remember.
What bait are you using? and What do you normally catch?