You say it comes on once after you set the stat. During that first start do you get any heat or is it just cold air?
If you don't get any heat during that first attempt at all, are you sure you have the propane tank on and you have purged the air from the line?
The blower will always start but will shut off if it does not ignite.
2015 Chevy Silverado 2500HD CC Duramax
2016 Eagle 323LKTS Java
Pull Rite SuperGlide #4100 Hitch w/Mor-Ryde Pin Box