In some cases, when adding function buttons, editing the window, or increasing the font size, some SAP buttons disappear, like Save/Ok or Cancel -buttons.
There can be multiple reasons for this, but here's a list of the most common issues:
1) You have not restarted SAP B1 Client after the font size change, try to restart the client and see if you can still reproduce the issue.
2) The form size is set for too small in the Item Placement Tool or Function Buttons forms. Remove the width & height from the configurations or adjust the size (see the video at the end of the article) and see if you can still reproduce the problem.
3) You're not using a resolution required to view the full document. If the document cannot be shown on the screen completely without scrollbars, you need to create buttons manually with the Item Placement Tool instead. To create a button in IPT, just select "Button" in the "Type" -column. This article gives more insight on how to create buttons with the Item Placement tool.
This video also explains the issue and how it can be solved.