The safety stem has to engage the cocking piece, and cam it back, to disengage it from the sear. You have some wear, so that the safety can no longer engage the cocking piece easily. The assembly will have to be dismantled, and the camming surfaces adjusted so that the safety can engage the cocking piece properly. Usually it is necessary to alter the edge of the camming surface of the cocking piece slightly, so that the safety shank can act upon the cocking piece. Because dependable safety operation is critical, it might be better to take the rifle to someone who has corrected this problem before. This pattern of failure of M70 safeties is not at all uncommon. The safety itself is small, with small bearing surfaces, and often doesn't fit in the bolt plug all that well to begin with.