there are more things to look into and eliminate 1 by 1 .
1. What filter are you using? sponge or hob filter? try to low down water flow when they lay eggs otherwise strong current/flow will wash male sperm away.
2. Try lower TDS. (eventhough eggs may hatch in high TDS but more chance in lower TDS).
3. Clean and wipes down the tank pretty good. you need to PP it if necessary.
4. Does male and female take turn fanning eggs?
5. Try meth blue
6. take eggs and hatch them yourself.