Hi,
I assume you mean a Hype Project with buttons using mouseover and mouseout and when previewing in the browser.
There is a known issue with how the browsers deal with these.
Vaguely remember people trying a few tricks to solve it, including myself
(I think when ever I ran into it I may have resorted in all mouseouts also ran code to turn everything off then turn the new one on )
Cannot remember if it has ever been solved to satisfaction and I have not seen it happen for a while.
It may be worth trying to search the forum for what people have tried.
for example