No way for others to tell at all without a ton of info such as when started, where they are, what they're in, what the plants are, temperature, light, watering, fertilizer, and so on.

THEN you'll get a huge range of possibilities depending on what people have run across on their own turf, so to speak.

Pics are very helpful as well.

Is there anything that can be used as a reference? A web site that shows photos of seedling progress perhaps? I started different seeds at different times. The heat, light, and watering have mostly been consistent for all. I do not have special grow lights. I do not plan to use any fertilizer. I started the seedlings in organic seed-starting mix.

What I've seen is that some seeds may germinate and then reach a certain stage and just wait for better conditions to really take off. In that stage, it may seem like they just remain about the same size for a really long time.

What you think of as adequate lighting for a seedling may not be, especially if you mention that you aren't using any particularly special set up for growing them.

Another observation is that, when having seedlings or even 4-6" plants that remain in their starter container, they also didn't seem to grow as much.

But put them out into larger pots or in-ground, and they will start really growing also.

Different seedlings grow at different rates. Don't worry about that. Just make sure they have enough water, not too wet; air circulation; enough light so they don't grow long, leggy stems; and feed them with 1/4 strength Miracle Gro powdered fertilizer.

Don't cover them. Air flow is important.

Go to Google. Click on images. Search for Seedling Development. There are many pictures.