Well, there's just one thing left to do... transplant into different soil.
I have never ever come across a thread regarding soil with added nutes that referred to this type of soil as useful. Everybody has had nute burn because of it. Plus, the NKP of nutes in soil are never useful, for seedlings, vegging or flowering.
Flushing actually makes the problem even bigger. The more water you add, the more nutes will be released and get to attack your plant. Many have killed their plants through flushing.
If you can't change the soil, try adding only as much water as is needed by the plant, not a drop more. This will lower the amount of nutes freed from the soil. Your plant will eventually grow enough to be able to handle the amount of nutes from the soil. Still, this is not a guarantee, your best bet remains transplanting in normal potting soil for flowers without any added nutes.