Hydration might certainly be your problem here but I'll also add something that a lot of people never talk about nor even know it happens. When you are lathering your first pass it will always seem thicker than the 2nd or 3rd because your stubble is longer and holds more of the lather to your face than on the 2nd / 3rd passes... I used to bowl lather but didn't really notice this until I started face lathering full time. Its very deceiving from 1st pass to the next but more pronounced when you face lather...Thanks. I thought that hydration might be the problem, the lather is acceptable in the first pass but thins out easily in the next two passes.
The brush seems kinda empty after finishing the second pass, if I load more I must face lather all over again and it doesn't feel much comfortable on a freshly shaved face. I did it once, although the tuxedo is soft I had some irritations and the lather became more thick than I needed to be, so I couldn't catch the left over hairs!
The Yaqis that I have (5) are all excellent values!
I am an exclusive Stirling user and like I said I used to bowl lather and changed about a year and a half ago as I actually feel I get more satisfying lathers overall now. I use 3 synthetic brushes which I purchased knots and handles independently from different vendors except for a Whipped Dog synth I purchased a couple months after I started traditional wet shaving in late 2016. I've learned what works best for me is during or after I have filled my sink with water I will splay whichever brush I'm using under water, squeeze and wring the knot while pushing the knot into my hand between my thumb and index finger so as not to pull the knot out of the brush, and then proceed to load the brush out of one of my Stirling tubs. Circular strokes for about 15-20 seconds is all I need as I'm only building enough lather for 2 passes which seems sometimes like I have actually enough for 4-5 passes usually... I then move to my face and splay and with circular strokes I start to build the lather and 2 or 3 times in between I will just dunk the tips of the brush only to pick up water to help hydrate the lather. This is what has worked best for me in my experimentation since late 2016.