Will not hold a zero - Hi-Lux Optics Tac-Dot 21x16mm Tactical 4MOA Dot Red Dot Sights w/Integral Mount

bought this for my ruger 10/22 sighted it in and used it that day, put it i the safe and several months later pulled it back out and it lost zero by A LOT, i re-sighted it in but it keeps losing zero. i can no longer return it because i didnt know until it was passed the point of accepting returns. now i have a 70$ paper weight.