I am having a similiar problem with my dwarf sags as well.

My vals have shot out tons of new sprouts, my ozelot swords are growing many new healthy looking leaves and my pennywort and assorted stem plants are flourishing. However, my dwarf sags have started to turn a yellow and I only have two of them producing shoots. The only other plant that I may be having issues with is my crypt lucens which appears to be turning yellow as well (this may just be the color as crypts change in different tanks and the leaves feel sturdy), and an aquarium lily that was damaged in shipment and now all I have is a bulb that likes to grow algae. I thought a nutrient deficiency would show up on my stem plants first since they grow faster. Is this a wrong assumption?

I thought it may be light since my dwarf sags are well dwarf but my lower portions on my stem plants are still thriving, and my new val sprouts are growing well also.

I have only been set up for two weeks but all my other plants seem to be thriving and I heard dwarf sag can almost be invasive it grows so fast.

Any ideas? Are Dwarf sags more sensitive to anything? I'm not at home otherwise I would post a photo.

All I have outside of plant life is a bulldog pleco, 6 amano shrimp and I just added 9 danios yesterday. I'm waiting on my LFS to get some oto's.

I am running a low tech no co2 no excel 75 gallon.
Lighting is 4 40 watt t-12's 6700K
I dose per the non co2 methods article with the exact products mentioned.
I only top off evaporated water which is about 5 gallons a week or just under.
I use Seachem prime as a water conditioner and probably overdose a little since I only treat 5 gallons at a time.