Can't seem to screw sprinkler head & tube back into the part in ground. It's there another part in between?

Answer 1

if you are replacing the whole assembly (which is preferred way to do if you already dug around it), you would check to see if the riser tube is broken. It is a small nipple tube (has male threads on both ends) that sits in between the PVC fitting and the sprinkler assembly.

Answer 2

You probably need to dig down to see what you are trying to screw the riser pipe back into. I wouldn't be suprised if rock or dirt got into the part at the bottom. Dig around and clear out the dirt from this, wash off with water, screw the tube back in and then replace the dirt/grass.