Well that is just plain weird. I've used freezer paper for years to make stencils and masks for fabric painting and I've stuck it to all sorts of fabrics - including knits - with zero troubles. It sticks down tight enough that I can paint over it without any leakage. If you start stretching and bending and pulling around on the fabric it will let go (especially on something like a knit), but if you keep it relatively flat it SHOULD stick quite well.
Pressure, maybe? I tend to have a heavy hand, maybe you need to press harder?? Is your ironing board really cushy? If so, maybe try pressing on a harder surface?
What makes freezer paper stick is that you're actually melting the plastic layer just a little bit. Not sure that info really helps, but I suppose humidity could be the culprit.(??)