My short-term memory has gotten noticeably less reliable in the last few months. My first question is whether you smoke pot. That's a major reason for decreases in short-term memory among younger people.
Stress also can cause major interference with short-term memory.
A preoccupation with troubling problems or changes in your life could cause you to fail to register things that seem less important. Many people like to use smart drugs, which are supposed to boost memory and concentration. Smart drug enthusiasts also think DHEA works to protect brain cells from the degenerative changes of old age. There really isn't much evidence supporting the effectiveness of smart drugs, though, and we don't have information on their long-term use. I meet lots of people who take Ginkgo biloba for memory, and I sometimes recommend it for older folks who may have impaired circulation to the brain.