What do you mean by "sometime it doesn't"? Does the text not fit on the screen?
The optional split tells OpenLP where you would like the verse to split IF it does not fill all on one screen. If the verse does fit, then it won't split the text. If you don't use an optional split, OpenLP will still split the text, but in this circumstance you have no control over where OpenLP will split it.
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If you want to FORCE OpenLP to split a verse on to two screens create another verse with the same name.