Several times i've had dropdown list dissappear?

[Dibbley247]

I have a workbook with 4 sheets that use dropdown boxes. I few times now, the dropdowns will disappear? so i have to copy/paste and adjust from another sheet?
Does anyone know why this happens?

I have a feeling it's since our workplace have changed to these fantastically slow VDU screens which is basically a screen which connects to a server so everyone has access to the same stuff?

Any help is good, Thanks

[mrice]

Validation can disappear if values are pasted into the validated area rather than typed.

[Dibbley247]

I would understand that, but it's every single one on the same page, and half of them wouldn't even be used, so there wouldn't be any pasting.

However, at the end of the week, a macro will clear the contents of these cells ready for a new week once a copy has been made. But wouldn't this happen all the time rather that what seems random?