Might not be enough light, both will want some direct sunlight even when very small and that is hard to mimic with fluorescents. How many and what power lights do you have? What length? How many pots? How far are the lights from the surface of the soil? Any natural light? How long are the lights on each day?

Nicotiana are very small when they first germinate and don't do a lot for the first week or so, then they should grow steadily. I sow Nicotiana ten weeks before I expect to plant them out and expect them to be good strong rosettes several inches across, depending on how much root space they have. They don't really seem to take off until they get planted out.

Maybe they need some fertiliser?

SnB,
I have them under double-bulb 40W cool white fixtures. The top of the plant is about 3-4 inches from the bottom of the bulb.
They get a small amount of sunlight for an hour or so through a window in the morning..
They all have 1-3 true leaves that are the same size as the seed leaves.

I suppose it varies by the plant species, but in my experience you can get a lot closer than four inches.

Essentially, with light-hungry plants just take care that the plants don't get burned by touching the hot tubes and then give 'em all you've got.

My seedlings right now are 1/2 inch from the tubes.

Are Nicotiana and Dianthus "light-hungry species"? Does anyone have some document describing light requirements by species? My Marigolds in the same conditions are starting to flower already!

A quick google search shows they are listed as 'somewhat' shade tolerant.

Tailor the light to the plant. If it's weak, it's either too cool or too dark, maybe both.

If you raised healthy Marigolds under the same conditions, light isn't the problem. Think soil, water, or fertiliser.

Interesting. Soil is exactly the same, water is the same, feeding schedule is the same. The only thing I can think of is that since the roots are much smaller, they have access to the fertilizer (liquid) for shorter time. Should I try feeding more frequently? When I feed them I can give them a very small amount, just to saturate the roots instead of the whole pot.