When I say rhythm I don't mean the rhyme scheme so much as the pattern of stressed and unstressed syllables. This is a problem that I could see with writing haiku's in English. Because the form was developed in a language so different from English, it doesn't really deal with stressed and unstressed syllables in the same way that a traditional English language form would. Also, rather than just rhyming you should practice with consonance, assonance, alliteration, etc. There are many devices which poets use to effect the way words are read and spoken. It's good to practice using them all.