Are the negatives thin in the entire spectrum of grayscale? Or just the shadows?
If it's only the shadows, you need to increase your exposure and develop for the same amount of time or possibly a little less. If it's the whole grayscale, it's probably a combination of underexposure and underdevelopment.
You need to be very observant and take notes so that you have records and can backtrack through them to understand trends in your development. For these notes to be useful, it's best to learn with one film and one developer. Print your negs often too, or (God forbid
) scan them if that's what you do. Learn how to change exposure and development to suit your needs, to generate negatives that print easily with a full rich scale.
Increased agitation and development time can and will alter how your negatives look. As will exposure. A simplified version of the full truth (a good place to start) is to judge your shadow detail (also called film speed) by exposing your film at one stop over (for ISO 400 film - exposure index 200), normal (400), and one stop under (800). Develop normal according to developer manufacturer's instructions. Then print/scan your negatives to find out at what exposure level you get sufficient shadow detail, and then learn the lesson that if you underexpose you negatives, especially in situations with a lot of contrast, you will sacrifice shadow detail.
When you get the shadow detail right, (don't pay particular attention to the rest of the tonal scale until you're done with determining shadow detail / film speed), it's time to fine tune development. Shoot a roll at your preferred film speed / shadow level - this is an individual number for you and you only - and now cut the roll in three shorter pieces. Develop one -25%, one 'normal', and one +25%. Now repeat the printing/scanning and see where you like the whole tonal scale.
With time you will learn to judge how you must expose and process your film in order to get a good full range negative from all kinds of lighting situations. Agitation is a powerful factor, and I change it from agitating every 30s to every 5m depending on what I need and want from my negative. With altered agitation, total development time needs to be adjusted as well...
So, experiment, but don't experiment with materials. Shoot and process lots with the same film/dev combo (LOTS - as in a couple hundred rolls). Take notes. Study your notes. Print often. Be curious. Break rules. Completely overdevelop a roll by adding 50% time to your development regime, or agitate the hell out of the film tank. Just to see what happens. If you study closely what happens, you might just learn something you wish to use.
And, lastly, get a tank that you can invert fully if you can. It's better that way.