has anyone else had a problem fitting there`s?

i can`t get the shaft to (just) pop in,it goes in until it catch`s the splined ring then stops (hitting it in slightly hasn`t helped).

i can see inside the splined area and the splined collar seems to have very slightly dropped to the bottom.
it seems as thought the shaft isn`t lifting the collar enought to slip in.
any ideas`s?
cheers
dave