Sometimes, the drain pipe is perfectly clear, but the tub still won't drain in winter. The culprit may be the roof.
Go outside and look at the roof vent pipe for the bathroom. If covered with frost or ice, that’s likely the culprit. bathtub won't drain cold weather
The fastest way to diagnose the issue is to check the other drains in your bathroom. Does the sink drain slowly? Does the toilet flush normally? Sometimes, the drain pipe is perfectly clear, but