When I create a numbers document on my mac, then open it on my ipad, then again on my mac, some formulas disappear. Why?

Numbers for iOS supports only a subset of the functions supported in Numbers '09 (for Mac). If a function is not supported in Numbers for iOS, it will be dropped when the file is opened on an iOS device.

If Numbers for iOS does not support a function you need, place a feature enhancement request through the Feedback channel for Numbers for iOS. The link will take you to the feedback page or form.

Regards,

Barry.