There are a number of reasons why this could happen: differences between your harp at home and the one in your teacher’s studio, fatigue, distraction, illness coming on, etc. But another thing could be the way you are practising. Sometimes you can practise for hours, but the information is not being processed at the really conscious level, so it’s akin to spinning your wheels. Slow down and be very methodical and extremely conscious of everything you are doing. Notice patterns that repeat, chord structures, anything at all that you can identify and label in your mind. Read the music while you are away from the harp. Visualize playing it while you’re in a quiet place and your eyes are closed. You will be amazed at the results you will get.