Do you fill the potato containers when you first plant the tubers? If so try another way lots of folk find successful. Plant them first in a much shallower layer of compost, and add more as they grow.
Can't help with the onions, unless it was just the bad summers recently.

I do the same as Stera, fill each bag to about 6 inches and cover the tubers. As they sprout, top up as they grow. I push canes down the sides of the containers and string them around to support the haulms. As most first earlies flower (although few don't) I dig my hand in to see any progress. Water and feed regularly, which you do already. Perhaps try some different varieties; mine are Sutton's Foremost or Maris Bard. As for the onions; they are deep feeders so make sure that the soil is friable and deeply dug. I think that your problem is that they get very wet and then very dry so the skins set and they expand. Could good light also be a problem? In the past I have had success with Turbo, which is a good keeper but every veggie gardener will have their own favourites.

Hya Really appreciate your Feedback, i shall try doing as you say with the Potatoes, have tried covering them slowly before but never very shallow, so will do as you suggest,covering slowly and try another type, and see what happens, i think you might be right about the Onions, weather being the cause, as a friend who used to be a big Onion grower, said the same, but will try the ones you have suggested, so Thanking you for your advice with this problem, cheers