"we couldn't refresh the connection" when accessing data in excel table

[indygreg]

My situation
- I have an app with three worksheets, each with a table on it.
- the tables are part of a data model with relationships defined between them.
- on top of that data model i have numerous pivot tables defined.
- data is appended to the excel tables every day with a vba script

suddenly the power pivot link to one of the tables quit working.
- if i "manage data model" i see in the power pivot data that it is days behind the table in excel.
- if i refresh i get the error "we couldn't refresh the connection".
- in the "existing connections" it shows no connections found though the other two tables are staying in sync.
- in excel if i go to the offending table and say "add to data model" it tells me that the file is open.

kind of new to power pivot so i apologize is this is something obvious but i haven't found an answer by searching.

thanks

greg