I think you're right about compost a year on, Anne, and I can easily add a bit of Miraclegro or similar to the mix of older stuff to build up the nutrients, but the water retention doesn't seem to be affected this year.
My stash of JM is outside by the shed with limited exposure, but the top bag gets quite wet! I may just cover it all up this year, and risk the spiders and snails jumping out at me next year...