Plants persist without growing, or grow slowly and thin.
Short answer
Plant growth is limited by whichever input is shortest — light, carbon or nutrients. Adding more of the others will not help, and adding more light usually produces algae.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: A high-light species — dwarf baby tears, most red stems, carpeting plants — in a low-light tank.
How to tell: Bright light, good fertiliser, and plants that still stall while algae thrives. Classic light-carbon mismatch.
How to tell: Nitrate at zero, or no fertiliser being used at all, in a tank with adequate light.
How to tell: Planted within the last month, particularly a slow species or one converting from emersed growth.