2004 Mazda MPV not getting up to temp?? (According to gauge)

About a week ago I noticed that the temp gauge only rose about 1/4 up after driving it for about a half hour. Check the fluid, was a low in the overflow, filled it drove it again, no change in temp gauge level or fluid in overflow.

About 4 months ago I changed the thermostat and it was fine until last week. I am wondering if the temp sensor could be bad or perhaps the thermostats is stuck open??

After driving it for another 20 min the upper hose is hot to the touch while the lower is cool.. Heater heats like normal...

I am at a loss.. Any thoughts or suggestions?

Thanks