Whenever a gardener complains to me about disappearing mulch or compost, apparently hoping for advice or sympathy, I respond , "Of course. I want my mulch to disappear!"
Mulch and compost seems to disappear because it is decomposing and being consumed by soil life, thus being converted into something that my plants can use as fertilizer. So if it doesn't disappear through the seasons, I'd be worried that my soil was "dead". And if the mulch/compost doesn't degrade, then my plants and garden worms aren't getting fed. So I say, rot baby rot!
Here in the tropics the mulch and compost seems to decompose fast. Where my homestead is located, the temperature is constantly moderate and we get a decent amount of rain. So conditions are ideal for the soil microbes and the worms. Add to these facts that I don't get cold winters, my organic material gets the opportunity to decompose 12 months a year. The process never stops. When I gardened in NJ, the growing season was much, much shorter. So mulch lasted "the season".....which actually was only 3 months for most crops. That's no different than how long I expect my mulch to last here. It's just that in New Jersey mulch only needed to be applied once, sometimes twice during the year. Here in Hawaii where I can garden year around, it needs to be applied at least four times a year. It just seems to be more than usual, but it's not.
I rake or till a bit of compost into the soil between each crop. Some of the old mulch also gets worked in. Then I apply fresh mulch. This cycle repeats itself with each crop. Thus each new crop is being fed by the decomposing mulch from crops before them.
So rather than bemoan disappearing mulch, rejoice that the system is working. I do. I'm glad to see my mulch decomposing. It means that I never need to buy expensive store bought fertilizer.