Dulcimer tuning problem

Hi. I have a McSpadden dulcimer which I purchased in November. I am a beginning player and have enjoyed it so far. However, after not being able to play much in the month of January I have started practicing again. If I am playing in DAA tuning, all is well. If I change to DAD tuning, my top melody string keeps going flat - even after one song. Could this be a problem with the string, or possibly with the tuning peg? Just looking for some answers. Thanks