Mailmerge

[Barquentine]

I have a spreadsheet in Excel as the data source for some letters. In some cells I enter text appropriate to that addressee.
When I merge to a new document only a few lines of the text appear - the rest is invisible.
I tried breaking it into two adjacent column cells - which improved it a bit - but it seems after a certain amount of text (about the same amount in h cells) the rest doesn't merge and is not visible.
Any ideas if there's a limit on how much you can merge from an Excel cell into a mailmerge doc?
many thanks.