Abandoned blue tit

We have a nest camera in one of our boxes which a blue tit nested in. All the eggs hatched and all chicks fledged apart from one. I left it alone but noticed the parents had abandoned it and wernt returning to feed it. After watching it continuously i rescued the baby blue tit and brought it inside in the warm and started to feed it. It has now been over a week and i have been taking it into the garden daily hopeing it would fly away into the garden with the rest of them. Unfortunately it still hasnt left and im now worried it either cant fly or there is something else amiss. Has anyone got any advice please as i cant find anything at all online to help and i just dont know what to do as it says online they should be fully independent as 4 weeks old?