layering soil can be a problem. In the past putting gravel or shards was recommended for improving drainage in pots however mixing soil and potting mix doesn't not work as well. The soil, especially if it has clay and the potting soil gets very heavy and holds on to a lot of water in a pot, when that stuff dries it contracts so that when you water, the water flows around the sides and out the pot but the root ball over time will get drier and drier. It should take longer than a year to do that though.
Usually in pots you only use potting mix or peat moss and the drainage material is mixed in not layered at the bottom.
Try soaking the pot if it is small enough in a tub of water. If a lot of air bubbles come out then the center mass was actually very dry. If it is adequately wet some bubbles will come through but not as much as when it is dry.
If it is not a case of the center core being very dry, it can as Tom suggests be the other problem when you mix clay and potting soil together it can stay wet too long and cause root rot.
I plant my citrus trees in pure cinder, no potting soil. It drains very freely and and I have to water every day and fertilize since cinder has minerals but not a lot of nutrients. What is does have is a lot of air space for the roots to fill. The pots are large, 20 inch diameter, and very heavy. The oldest tree I think is about 19 years now and it has never been repotted. If I don't water it for four days, the leaves will start to curl. If I had planted it in potting mix and cinder I would have had to repot it every couple of years or I would have the issure of the dry core and the pot is too big and heavy to soak it.
I did have a bougainvillea in a 14 inch pot in pretty much dirt. Every couple of years I had to take it out of the pot, poke holes in the core so air and water could infiltrate and shave off about two inches of roots from the sides and whatever was possible from the bottom just to keep it in the same pot. My neighbor saw me doing that but did not realize that bougainvilleas actually are hard to transplant and ended up taking most of the soil off the plant and it died.