low heat output

my 08 , 4 cyl, auto was losing heat temperature. Originally when the temp dial was at the half way (blue/red) point the temp was high enough to sometimes being too high. The temp diminished over a few weeks to being cool at the same point and at full red was about was warm as 1/2 way had been. The temp gauge was always in the middle. The thermostat and coolant was replaced with no increase in heater temp output. The work was done at a local shop, not the dealer. Any ideas why I lost heat ?

 

Sounds like it could be your thermostat was dying and now is dead. I had a similar issue with my Ranger a year ago and replacing that got things back in order. You should be able to check to see if warm/hot water/antifreeze is running through your lines. Be very careful though as you don't want to get hurt in the process of checking. Start with a cold truck and check to see if warm/hot water is flowing to the heater core.