'Undo' not working properly

I create my own toolbar for PowerPoint to do something like insert template
slides, or insert picture from template slides. But after I did couple
actions by using my toolbar, I press 'undo' from native office menu, it
simply undo all the actions that I did before instead of one by one. And When
I tried again, and go look at undo stack, there is only one action in the
stack no matter how many actions I did by using my toolbar.

Really don't know why does that happen?

Can anyone give me direction on it, thanks in advance.