Application, and/or expectation issue. Regular water based inks (other than white), do not make a film layer on the shirt, but rather soak in and color the fibers. When you stretch the shirt, it exposes the valleys of the weave where the ink did not reach. It's not that the ink broke/failed, as this type doesn't make a solid film.

You could give it more strokes/hits of ink. Or--uhm, don't stretch the shirt It would be less of an issue with finer, smoother fabrics, much more of an issue with something like a ribbed tank top.

Opaque water based inks (like white ink or the SuperCover line) form a film on which you can do a stretch test (as would all Plastisol inks).

 

I’m using speedball water based ink and a 110 speedball screen

Is this a curing issue or an application issue like it’s not pressed deep enough into the grooves?

These inks are not really intended for loose weave or ribbed fabrics.
It is still possible to do by pushing the ink all the way through, but it's a messy and difficult process.

You can try using a higher mesh screen, thin the ink to the appropriate level, and do 2-4 print strokes.
In reality only the first stroke is a print stroke, followed by 2-3 incremental pressure strokes to push the ink trough the fabric.
You can do a second pass to apply a bit more ink if required (without flash cure), followed by another 2-3 pressure strokes.
You can also use a "flattening screen" to push a bit more.
Obviously this is not an easy process to do in a manual press, so it is better to avoid this type of fabrics.

My suggestion would be to buy some Gildan Softstyle tees and practice with them.
They are cheap and print well.

Regardless of the above, the gaps are sometimes unavoidable, and depending on the design they are in some cases acceptable.
Here is an example from a well known brand.