Why is my Grill not heating up fast and the flame is small...?

Well today i was getting ready to have a BBQ and then i noticed i had almost no propane in the tank so i went to get a new 1 and what not anyway, i hooked it up and stuff then i turn the valve on top and start the grill i started the middle and back burners on also. after a while i was getting ready to put stuff on and it was only at 250 degrees after 20 minutes and last year it got to 400 in like 15 so i was wondering is there any real explanation for this problem? Also the flam is VERY VERY small even on the high settings an if i open the grill for a quick second the flam would go out.and every valve was opened al lthe way and everything....