If you go up in needle size, you will surely get even fewer stitches per inch. When knitting a gauge swatch, it should always be at least 6x6 inches in size, with the outer edge stitches in garter stitch... Measure the 4x4 inches in the center of the swatch. The edge stitches can give you misinformation, which is why you make a larger than desired area to check gauge.
Yup! Done all that with garter stitch border! It’s a mystery as to why having gone down a needle size I actually got fewer stitches. I normally need to go down a size as I’m obviously a loose knitter.....never got fewer stitches before. And the same thing happened twice so not a fluke?