Mine were the same when I put them out in the hot weather at end of June/beginning of July. They just buried themselves and stayed buried until one night I started to worry and was out there at 10.30 digging around trying to find them. I bought them indoors after installing new separate heating and lighting (the Exo Terra Solar Glo combined MVB blew for the 3rd time in 5 months) and they are like totally different torts; they are active, stay out for almost the full 12 hours that the light and heat are on, and have even started humping each other lately!
I had always planned to put a shed next to their outdoor pen so that they can come and go as they please, and this was delivered this Wednesday, so am now in the process of treating it before putting it up and then insulating it ready for next spring. My plan will be for them to be out there from Spring to Autumn (depending on weather) and I bought one of the small Stella Beer fridges at Easter to hibernate them in (storing fridge inside the house) over winter.