It's hard to say how long it will take for your VFTs to turn red. It could be days or weeks. The more sunlight they get the faster they'll turn red.
Where the flies still alive when you put them in the trap?
The trigger hairs inside the trap have to be stimulated after the trap has closed on the insect. This is usually done by the insect trying to get out. Otherwise the trap will reopen in about 24hrs. In the wild this keeps the trap from wasting energy trying to digest sticks or leaves that fall in them.