Yes wash in Nikwax does work..
Better than the spray on stuff
And you have to use heat after to fix it.
Depends on the waterproof and how valuable it is to you really. .
Even sending them off to be reproofed can be worthwhile
You'll rarely get back the same dwr and 'beading' that it had when new, but you can extend its life, a lot.
Oh and looking after it (not crushing it into a bag, wet or filthy then leaving it to moulder for days, for instance)
Or even left hanging on a hook where it gets lots of passing traffic won't help either..