First check for bugs. If no bugs check your ph in. Looks like a cl/mag lockout. They work together. If ph is off and 1 of them stops uptaking you will see deficiency from both. Just what i see from either deficiency or lockout of these primary nutrients in my grows. Simple start. Bugs, ph,ammend.
No flush. Adjust. You are far in and a lot of recovery will not occur. You will get over the finish line. A good indicator of flower health will show at flower tops. Nice healthy green will show flowers uptaking. No panic. That plant will finish just fine.