Normally.. it’s any point of friction with the strings. First though... make sure you know how to put on strings correctly and stretch them in. I’ve had to show gigging guitarist with 30 years experience how to ”lock” their strings. (They also tend to “deck” their trems.) Once that is stable.. pencil lead in the nut slots and on the bridge saddles, then a tiny spot of oil on the string trees helps a lot. BUT.. I learned a long time ago that I don’t know everything. A PRO setup from a reputable shop is well worth the money. That said.. I usually have to adjust their adjustments. Also. every Strat is different.. some are more stubborn than others. The more they are played.. the more they cooperate.