Sometimes letterpress printing can be like riding a bike while drinking a cup of coffee, while balancing an egg on your head, while writing a letter. There are a lot of things going on, and a lot of things that always need constant attention.
One of those places is inking.
Proper inking is the cornerstone of quality letterpress printing. Too much ink and your print becomes fuzzy and undefined, too little ink and your color will begin to lighten and provide uneven coverage on large solids. For perfect letterpress printing you need to find the "Goldilocks" amount of ink for each project, and work to maintain that level over the entire project.
In letterpress printing inking is controlled by two factors - the physical amount of ink you put on the press and the height of the ink rollers as they pass over the printing plate. Proper height of ink rollers can be set with the aid of a roller height gage (often referred to as a "lollipop"), and can be checked under magnification (especially on jobs with finer details) with a loop. What may work for one job, may not work for the next, so it is important to make ink adjustment a priority on every job. The last thing to look out for when it comes to ink is color consistency (apart from mixing the color...that is a lesson for another time). Depending on what color was used last on the press, among a variety of other things, the color of the ink may change during a print run, so always keep your color guide close at hand.
make it a routine
Get a routine down. Checking the inking as you are printing should become second nature and automatic through the entirety of a press run. Three places to always check inking, every time...are on the image or design, in the negative space, and on the back of the paper. Write it down, make it into a song, just don't forget!