My tap water is around 400-450 TDS and I get spawnings in this however only around 5% will be fertile, while if I spawn the same pairs at around 80 TDS I get 95% + hatch rate. Discus parents will eat eggs that are infertile, I would try lowering the TDS to around 100 and see what happens with the next spawning. You may need to go lower than this, you may be successful at a higher TDS than this, it depends what proportion of your TDS is carbonate hardness.
It's also not unusual for young parents to eat the first few spawns until the get the hang of things, so it doesn't mean you've done anything wrong. Give them clean, soft water and a good diet and be patient, I'm sure it will come good in time