There are only so many variables you need to deal with at one time. Agitation, developing time, and film speed rating are the biggest ones here. Of course there are plenty of variables you could mess with, but it pays to change one or two things at once and then re-evaluate based on your results. If you change ten things at once, you'll never be able to sort out what did what.
For the moment, I'm assuming you are working with freshly mixed developer and enough of it. Old or exhausted developer can let you down, especially when using a dilute mix.
I'm assuming you had enough developer. Having just enough developer to barely cover the film can lead to killing the developer during the developing process, especially when using a diluted - think 1:1 or 1:2 etc - mix. There just isn't that much developing agent in that few ounces of liquid, so don't scrimp on filling the tank. Fill it to the brim.
I'm assuming you are keeping tight control over temperatures. Even a degree or so makes a difference.
If highlight density is low - flat negs -try 50% longer time than what you had. Call it 20 minutes in your case. Yes, that's a honking huge jump, but if you try to sneak up on it in tiny increments, 5 or 10% at a whack, you will be testing over and over and over and spend far more time than you want. Not to mention that you wouldn't be writing in here if the negs were just a teeny bit flat. Best to try to straddle the "real" time with really big increments, and then you can interpolate quite well by judging visually.
You didn't refer to the shadow areas specifically, but if the shadows look underexposed in your first test roll, try adding a stop of extra exposure.
Since some folks do verrrry minimal agitation, you are probably giving enough agitation.
Remember the old adage that you expose for the shadows and develop for the highlights. Think of them as pretty independent variables. 1. Give enough exposure to beef up shadow density to where you want it - rate the film speed to where you are getting usable detail in the shadows.
2. Develop long enough to build highlight density.
3. Examine your negatives and see if you have too little or too much density and adjust speed rating for shadow density, and adjust developing time to build enough highlight density.
4. Do a test print the way you usually print (or scan if that is how you print) to see how your exposure and development scheme works.
5. Based on your results, go back through steps 1 through 4 until you get workable results.
6. Keep detailed notes as you go so you don't lose track of what you did. Just don't ask me how I know this.