Why does the pitch accent of nouns seem to change in compound words?

I’m a pronunciation-focused learner who records myself daily to track progress, but I’ve hit a wall with pitch accent. I’ve noticed that words like 'ame' (rain) and 'kasa' (umbrella) change their pitch pattern when combined into 'amagasa'. Are there concrete rules for these shifts, or should I just be memorizing every compound word in isolation? I'm currently using Chickytutor.com to check my recording cadence, but I want to understand the underlying theory so I don't sound like a robot. I am asking specifically about learning Japanese, not a generic study routine.