Once again chicken TV is better than anything on cable, satellite, or antenna.
How much time have you given them to get used to no tarp? Chickens don't like change. The best example I have of that is that when mine wake up to a white world they are not going out in that but if the snow falls during the day when they are already out they stay out. It's not the snow that bothers them but the change. After two or three days of staying in the coop some will venture out but they don't do that immediately.
Since they are OK on a cloudy day but not on a sunny day it's not just that the expected tarp is not there. For you to know that they have probably been given enough opportunities. So yes it is kind if mysterious.
I don't think what they are doing is hurting them at all. The reason I'd want to change is if it is somehow inconvenient to you. I agree with LizzyJo, I would not force them. Just give them the option to to come out in the sun when and if they want to.
I sometimes keep chicks locked in the coop only for a while. When I open the pop door to give them access to the run for the first time every one may be on the ground within 15 minutes. But I've seen some broods to take into the third day before the first one builds up the courage to leave the safety of the coop. Just be patient and let them do the work.