Are you able to feel the string with each of your three fingers? When I'm shooting a lighter poundage bow, I can feel the pressure being exerted by each finger. I sometimes put more or less pressure on one finger or the other, to see what happens to my shot. As I increase the pressure on the bottom finger, I can feel the bottom finger pull in the string more. Usually, I will work the three fingers back and forth a little as I'm settling in to my shot until I get to a distribution of pressure that feels right to me.
I also shoot 3 under and don't use a deep hook when I'm shooting my lighter poundage bows. I've never had any fingers slip off the string.
The only thing I can think of is maybe you aren't as aware of your bottom finger as you should be. Or possibly you had some injury to it that caused it to be weaker than it should be. In either case, maybe working it a little as you settle into the string might help.