there could be a couple causes…
#1. what type of yarn are you using? is it what the pattern calls for?
#2. What is your gauge? are you matching what the pattern states? you may need to change your needle size if you find your gauge coming out larger than it should be.
#3. I never pay attention to how long it says to go for on the foot once i’ve completed the heel. I try them on and then eyeball. Like the usual pattern says to knit as long as needed until i’m 4" from the end of the toe… ok, fine, but I have funny shaped feet (more square instead of rounded) so the rounded toe is always too short and i end up with blisters on my toes, tryin it on lets me see where i am and how much more i need to do (the fun with this is if someone sees you with it on, they may think you broke your foot LOL)