You might try turning the loop up the serving, the serving acts as threads in this case. What' s probably happened is the string has stretched, particularly if this is a one cam with the long string. On these the string stretches it' s entire length and since most of the length is above the nocking point it moves down.
Now for more, it sounds like you' re nocking the arrow between the knots of your loop. If this is the case the nock set should be above the loop. Either that or you' re positioning the arrow wrong, it should go below the nock set. Kinda hard to answer this without seeing the set up so hope this helps.