Is my vowel placement logic wrong before a final consonant?

I'm struggling with Thai orthography—specifically, why some vowels seem to flip or disappear when they end in a consonant like 'n' or 'm'. I've tried memorizing the script rules, but I keep misreading basic signs when I'm walking around Bangkok. Does anyone have a mnemonic or a better way to visualize these clusters, or should I just focus on learning words as whole blocks instead of focusing on individual characters?