Show layer on hover not working properly

I have a question slide where the student has to click to choose the nouns in sentences.  At the right of each sentence, there is a hint button.  If the student hovers over this button, a layer should show how many nouns are in the sentence.  I have five sentences and the hint shows only on the first sentence and usually only once.  What am I doing wrong? 

I have attached the file. 

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