based on my unlicensed but professional experience: it all depends on 3 things.
1. your character - this makes your amulets be effective or not, they multiply your energy basing on your character which is described by your astrology.
2. Merit that equates/negates karma - buddha even said, no amount of psychic power can refute karma, just remember maudgalyayana, he was killed, yet he was buddhas best disciple too, even though he had great siddhi abilities.
3. belief - its the fuel that empowers more so than less our samadhi, mantras, kathas, or amulet. Without it, multiplier or nuetralizer, it means nothings. And its one of the greatest denominator when it comes to results when wearing/applying any amulets. Ex. one can be protected more than the one who is not believing even tho they wear the same amulet by a monk - then again the amulet still has its power.
This is my experience tho, especially with love type amulets. As a matter of fact, even now I can attest that its easier to have protection/luck amulets to have good results than love amulets, but then again, that may be my karma too.