Why do my tone marks keep changing when I connect words?

I’m an absolute beginner stuck in a loop of trying different apps, and I’m finding the vowel elision and tone assimilation rules really difficult. I can see the tone marks on isolated words, but as soon as I try to form a sentence, the pitch rules seem to shift. Does anyone have a systematic way to memorize these sandhi-like tone changes in Yoruba, or am I overthinking the written rules?