SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

if you tried to do a quick change,meaning you just dug around the top of the head and unscrewed it,there is probably dirt around the elbow at the bottom. If you dig up the whole head all the way down to the elbow,if you dig a little below the elbow,you may be able to turn the elbow upside down so the hole and the threads face down,and the residual water will flush the dirt out of elbow. Or just dig it out the same way,turn the water on for a second to flush it all out. If the old head screwed off NO PROBLEM the new one will screw on no problem,unless of course there is dirt.
Another possibility, all i do is quick changes,but you cant use a hunter pgp to change out a rain bird 5000 that is in the ground. Reason being there is an extension on the bottom of the Rain Bird head,and the bottom of a pgp is flat, so the elbow is below the actual bottom of the head,and is surrounded by dirt. maybe check that out.
Other than that,just dig it up completely,turn water on to flush elbow and clean threads off,re-install new head.done deal. worst case scenario you need a new elbow that you can get at lowes or home depot.

A lot of times there is a 3/4" x 1/2" Thread x Thread Bushing that is also installed. Check your old head and look for a white fitting at the bottom of your head- unscrew it and install it on your new head

There's also a stupid swivel fitting out there that allows the head to just keep spinning, but you would have to hold onto it to take the head off so I doubt this is it.