Hi and welcome to Knitting Help!
You're doing the right thing going up in needle size. Be sure that you're doing a gauge swatch that will measure more than 10cm across. Cast on should be at least 40-45sts so that you can measure over the middle 10cm. That will give you the most accurate gauge.
GG is right, the yarn you're using and the one recommended in the pattern will also help. (Possibly this is a Dale pattern in Baby Ull?)