Assuming that you achieved the tune in the first place, the culprit will most often be something we are doing that makes the arrow not go to the spot we want. Like my friends like to remind me, "it is the loose nut on the bow", most often refering to me.
That bow will only change a little, we change a lot. Our mental attitude, fatigue, and variations in our bodies response to our enviorment.
I have experience what you are talking about, I don't have it as often as I once did, but I found that when it happens I am far less likely to change things in my equipment.
I most often will look at my form, and I will be darned if I didn't find something I was doing wrong.
Maybe they are right, it was the loose nut on the grip on the bow again!