I started a pair of Fair Isle red white and green socks for my Christmas in July theme. The first time I ripped them out they were way too small. Check gauge and try again. The second time they fit up until I started working on the foot(cuff down). They didn't fit over my heal. I checked my gauge again changed needle size and I'm now trying them for the 3rd time.
Real Fair Isle is tricky. Unlike yarn that is dyed to look like Fair Isle you are working with 3 different balls of yarn(or how ever many different colors you are working with). You have to have the gauge almost a little bigger than you want so that it doesn't end up too tight. You also have to make sure that the strands in the back are not pulled too tight. They need room to float and stretch so the item you are making is not too small. If the stands are too loose you also run into problems.
I have made other Fair Isle items like purses,hat and mittens without much trouble. I love working with different colors and patterns.
I will stick with the Christmas socks and I will win out in the end or find someone with smaller feet.