This happened to me last summer.
I called HL and asked them about it (the fins coming off). They said it happens a lot. They promised to send me some new ones when they came off backorder. I called several months later and they were still on "backorder". They promised to send them when they came in from the factory. Haven't heard from them since.
The board pretty much sat for about 6 months until I was able to find some from Malibu Dealer's shop in Dallas. I put them on and actually glassed them to the board (with surfboard repair resin). They haven't fell out since.