Plants are in soil that I try to amend with compost and then reuse. I add fertilizer about every 3 waterings at about half the suggested amount.
They are getting worse every day. Leaves turning yellow with green veins, so gave them a shot of fertilizer for the nitrogen. Didn't help.
What am I doing wrong???
I agree with the others here, that all you can really do at this point, is let the soil dry more before watering again.
The term you're looking for is "chronic overwatering".
It's the fastest way newer growers and old kill their plants.
But alas, the problem often isn't really the growers own doing, but the soils or substrate.
It's all aeration and drainage. All it is.
How often a grower waters, doesn't explain the "why" or underlying reason for chronic overwatering.
Some hydroponic grows are watering more than 6x per day.
You added compost to reused soil when you amended.
Compost holds onto a helluvalot of water.
Did you add any aeration to your reused soil when you amended?
Regardless, adding some perlite to your pots next-time will help a lot. They're not overly large.
In my own opinion, chronic overwatering shouldn't really exist for the average grower, unless pots are far too large.
Nothing you can really do now. Besides watering less often. Letting it dry more between waters. And run-to-waste each watering.
Next-time around, before potting up, mix in 20-30% perlite or so. That's my advice.
You'll need to water more frequently, but it's so much more forgiving.
Roots crave aeration all day.
GL.