Each plant will have its own needs. Some like more nutrients, others less. Some don't mind ph fluctuations, others do. It would be nice if there was a single set of instructions for growing cannabis, but the fact is, there will always be variations. A good grower will be able to assess the individual needs of the plants, and treat them all accordingly.

Flushing in this case wouldn't help, yet, until we know the problem. You need to find out the ph of the water going in first. If it's not in range (6.0-7.0), you'll have to adjust it to range, then water until you get run-off, and measure the ph of the run-off, and make sure it's in range also. (If your run-off is not clear, run it through a coffee filter or paper towel first when using ph strips/drops, the color of the water can skew results)

Once you know that ph is stable, you can try feeding at 1/4 strength veg nutes and work your way up as long as there is no tip burn. You won't see results immediately, it'll take a few days to a week, maybe more, to get that green back in them.

If the ph is off, giving nutrients won't help. Also flushing will make things worse if the ph is off to begin with.