Need HELP with door handles

Hey guys, i been having a problem with my door handles lately, and i dont know what to do. The problem is that i have an alarm system and it has a button that when u push it it locks the doors... but one of the doors doesnt lock... i always have to use my key to lock it... i went to the dealership and ordered new door handles.. it costed me 100 bucks ... maybe that will solve the problem? do u guys have any suggestions on how to install them? because last time i tried installing them a piece broke off from the back and then the handle was a waste... any suggestions whould be well appreciated... Thanks! :)

also i forgot to mention, when i try to lock the door from the inside its hard.. you have to use some power and force it to lock... i know its not supposed to be like that because my friend owns a del sol and his are easy to lock and unlock... anything whould help.. thanks