If you didn't do something to secure the thermostat it has probably pushed up and turned and is no longer blocking the water off to heat up the engine. Here's how I secure mine:
I pull a single strand of wire out of a piece of stranded wire,
Cut it to 3½" length.
and thread it through both sides of the thermostat like this.
Then drape the wires down the side of the neck like this.
Then push the hose down over it and tighten the hose clamp right at the top of the neck that the hose is pushed down on to be sure to catch the hold down wires.