I have experienced similar symptoms from time to time and the issue hasn't always been what you might think!
I use a fert that doesn't contain nitrate as my tap water is already high nitrate at 35ppm. Although nitrate is an essential macro fertilizer, I have found that some plants grow poorly if levels reach about 30ppm, so check your levels.
Thanks to my plants and the use of Matrix in my filter, levels in my tank were 10-15ppm for a long time then, about 9 months ago I started to get modest algae problems again. Due to high nitrate in my tap water I keep water changes to 15% per week and I started to wonder whether phosphate levels were creeping up as I had never tested for that. Well, I got a kit and was surprised to find that phosphate was zero - so phosphate was the issue, but a deficiency of it rather than an excess! Phosphate is another essential macronutrient so the plants will need some.
I now dose with a home made phosphate fert and aim to keep levels at 1.5ppm.
Well, that cured the problem and the plants shifted up another gear. It became clear that nitrates had stabilised at 10-15 only because phosphate had run out because once I started adding phosphate, nitrates fell again to 5ppm. They remained there for several months, but now I have a trace of algae coming back again. The reason seems to be because I have now run out of nitrate as levels of that are now zero!
As a result, plants have slowed a bit and I am not having to add as much phosphate as previously to maintain the desired level.
So, it's all about balance. Nitrate and phosphate are usually considered to be badies, but if your plants eliminate either of these completely they will stop growing well, and if you still have any of the other in the water you may get algae.
It's really hard to get this balance right and every tank will be different. However, I have learned that you may need to think counter intuitively - I can now see why adding phosphate resulted in a fall in nitrate, but who would have thought it?
My thoughts now are that I won't try to make up the apparent nitrate deficiency, but instead I will cut phosphates and aim for 1ppm and try to get the plants growing at a rate whereby neither runs out totally.