Can you upload your file or post a link to the website. It is kind of hard to tell what the cause is.
One thing that can cause problems is the position of the buttons. If you select your buttons and bring them to the top (Arrange > Bring to Front) this can sometimes solve the problem. If the buttons are lower in the layer structure than say a rectangular outline, even if the rectangle has no fill, the buttons will not work as the plane of the rectangle has them covered up.
Also, make sure your buttons are linked. If a button is not linked, the rollover action will not take place.
Worth looking at.
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