Maybe they are not positioned well. If they are not completely within a layout part (header, body, footer, ...) they might not be showing correctly.
Maybe they are touching another layout part's border.
In what layout part did you place the button?
There's a known issue with placing them in the footer and this may also happen in other layout parts such as the header:
For More Information see: Redraw bug in FM12 on footer elements
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip