Please try this. I've tried it on a brand new file and I'm still not seeing the bold on a rollover.

1. Drag a Button Shape widget to the work area.
2. Give it a label.
3. Right-click on the label and choose Edit Button Shape > Edit Rollover Style
4. Check the bold checkbox.
5. Click OK
6. Check the rollover.

I'm not seeing the text bold on the rollover.

Hi rem,

I think the interface may be a little confusing. After checking the checkbox next to Bold, click on the Bold button next to it to set the text to Bold. That should do it.