I had the same problem but with a thick mulch. I had a layer of leaves that I put down early in the season and watering did OK. But I got a load of chipped wood from a local tree surgeon and started having problems after laying down a thick (3-4") layer. Seems the wood chips would have to soak up as much water as possible before reaching the soil. I was watering as before but not much was getting to the soil. I installed soaker hoses under the mulch, in direct contact with the soil and now water once every 2-3 days as opposed to every day.