The arrows could make a difference as well...If the arrows are spined for 55 pounds and you go to 65 then they flex when absorbing the energy from the string, then they unflex during the flight...All this will slow the arrow down and affect it's flight...
All the components in shooting a bow either work together or against each other...I'd take it to a good bow shop as mentioned and have it checked out and see what they think about the current setup...