Macro works but buttons don't
 

I have an Excel macro that activates another application, then uses "Send
Keys" functions to access that application's menus. (The menu choices result
in exporting data from the called application into an Excel workbook.)

The macro works fine if I run it either from the Alt+F8 key command or the
Tools, Macros menu. But when I try to assign the macro to either a toolbar
button or a button object on a worksheet, it doesn't run all the way through.
It will switch to the other application, but only some of the SendKeys
functions seem to take place.

Can anyone offer guidance as to why the buttons will not work? Thanks in
advance.
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- markvi