Don't feel too bad. I had two female Carolina's last fall. Both mated with more than one male. Ended up with 8 ooths. Stored the ooths in my garage during the winter to keep them out of extreme temps. Kept them in a ventilated container and misted every couple weeks when it got dry. Made sure I had found and removed all Chinese mantis ooths from around my yard (to avoid competition). Attached the ooths to shrubs/plants in my yard. (north, south, east, and west exposures). And here I am on June 18 with no hatches at all. With all the humidity and heat over the last 4-6 weeks...I'm starting to wonder if I'll have any baby Carolina's this year. If the nymphs are small and anything like the European nymphs I struggled with, I ended up resorting to collecting and feeding them aphids and the smallest gnats I could catch. Even small fruit flies were just too intimidating!
Anyhow, don't get too down. Sounds like you're doing the right things and trying different approaches when it isn't working. Keep us posted!