i've been playing for probably around 15 years with very few lessons and am far from an expert. i would consider myself an intermediate player always looking to improve. i still find the F major to be more difficult than the rest but not too much of a problem anymore. 1 thing that i do find is that it is much easier for me to just finger the full F major barre chord rather than just the top 4 strings. i don't know why but it works for me. It may have to do with the positioning of the thumb and how much clamping action you can get. Also, as someone said above experiment with the angle of your index finger. a slight roll of the finger may make a big difference.