Bluetits activity slowed but still in the nest

As everyone's chicks are flying out of the nest, we are becoming more and more desperate. Our couple of blue tits has been busy with their nest throughout April, laying eggs at the start of May or so, and then at some point we saw a lot of very busy flying in and out with fat green tasty worms - we thought the chicks have hatched. Then, around May 12-13 I noticed there was no activity anymore. Male would come with a worm, come in the nest and then come out with it in the beak. No sounds or visible activity for a couple of days. I saw a great tit coming close to the nest and looking in, but it flew away. 

Now, the birds are still active in the nest - I see them both coming in and out a couple of times a day, I saw one of them bringing feathers when the weather turned chilly, but it is not the same joyful busyness. So I guess something went wrong with the nest, but why would the parents still be around?

Any advice is welcome, it is our first nest!

Thank you in advance.