There are a few possible reasons, but without knowing more I will mention some of the basic ones.
When you return a fish (especially a chub) then do it a little way down the bank. Chub have a habit of returning to the river and spooking everything else in the swim.
Chub tend to live in specific spots in the river. Under a snag, overhanging bank etc. You need to tempt them out to take the bait. They will learn that all their mates have been dragged out, and may become a bit twitchy. If you have reached a hard patch, possibly try moving down to the next swim where you think there should be some. Or lighten right up on your tackle, and change your bait presentation.
If you are using a keep net, maybe you have run out of fish to catch in that swim. Try moving.
Light is an issue. Chub will feel more confident in lower light conditions. If you are catching at dawn, then it tails off towards mid-day, then maybe that is it.
You have caught and eaten all the Chub