Hi,
There are usually only two things that will stop an engine from reaching it's correct operating temperature. First is if the fan remains on all the time, this is easy to diagnose and cure. Second is if the thermostat is stuck "open". Although this is a little harder to diagnose, the usual indications are low engine running temperature and that it takes longer for your heater to kick out warm air from the first start. It is easy to fix also but as you said, it's running ok.
Cheers now
Ken Chapman
May 2007