av8tor is correct in every part of his post.
Is it definitely the prop that is coming loose on the hub or is it the spinner cone coming loose?

You state it is a "tru turn type" spinner. I have seen many of the lesser brands of those where the spinner back plate is "not worth a toss" and the spinner itself is the cause of exactly the problems you are seeing.

Try taking the spinner off altogether (Including the backplate), Replace the prop and use a felt tip pen to mark a line across the hub of the engne and the prop hub. Fly the plane then when it has landed - see if the line is still lined up.

if it is still lined up - pick up the spinner, Place it on a sound surface (not your dining table - you will get in trouble) then smash it to pieces with a large hammer - then place said pieces in the bin and go get a better spinner