Is my epoxy still sticky because the room was too cold?

Yes, that could be a major reason. UltraClear Bar & Table Top Epoxy has to be applied and cured above 75°F, and the room must stay there around the clock for the entire 3-day curing window, not just while you pour. This is a requirement, not a recommendation, and there is no room for variance. An unheated shop or store that sits in the 50s or 60s during the day and drops into the 30s or 40s at night can be a major reason a pour stays sticky. Thermostats are not always accurate, so set yours a few degrees warmer.

Heat is a different matter. If half of a top cured and the other half stayed tacky in a hot, humid room, the difference is between batches: the batch that stayed soft was not measured or mixed completely.

UltraClear Art & Craft Epoxy needs 75°F to 80°F with humidity below 85%; it can be used in warmer conditions, which shorten its working time and pot life. For UltraClear Deep Pour Epoxy, the published guidance is 60°F to 85°F with humidity below 70%.

If the epoxy has been above 75°F for 72 hours and still hasn't hardened, temperature isn't the cause; the material on the surface was not fully mixed. Either way, epoxy that is still soft past the cure time will not harden on its own: coat over anything that is hard but tacky, and scrape off anything soft and gooey before re-pouring.