Excel macros occasionally do not run
 

I have a rather complex Excel vba application. It has about 10 sheets
each with 10 to 20 code modules. When the spreadsheet is opened, it
runs the Worksheet_Open event, which calls all of the subs as needed.
The entire app takes about 5 minutes to run. Occasionally I open the
spreadsheet and it goes straight to the sheets without running my
macros. It always asks to enable/disable the macros, this happens
after they are enabled. I have a refresh button on one of my sheets.
I can click this button and the macros run as expected. This is how I
know that macros have been enabled. Any ideas as to why this occurs.
thanks in advance.