I have some WW 375 H&H cases that wont a hold bullet in place even after FL re-sizing; they have been reload 5 times.

Will annealing solve this problem or is it time to toss them?

If the neck ID drags on the sizing button coming out you might try to reduce the sizing button just a little.
If the neck ID is not being sized by the button , than it is possible you are getting spring back in the neck diameters and softening the necks may help.
you might look at crimping. roger

I would also check the thickness of the brass to be sure the necks have not thinned out.

Is the FL die sizing correctly? Size 2 cases, 1 with the expander & 1 without. Measure the neck diameter of both. The 1 sized without the expander should be smaller in diameter. The expander should be .002" smaller then bullet diameter. Over working the case neck can thin the walls some, depending on the dies. Not common to not hold a bullet. Annealing is not a fix IMO.