Feeds import stalling when importing multiple worksheets in single XLS, or multiple XLS in a folder

Feeds Import is working great for importing a single XLS, but results in the import button on the importer page changing to "Importing (50%)" or whatever percent when I'm:

• Importing more than one XLS Workbook (with a single spreadsheet in each) in a folder
• Importing a single XLS Workbook (with multiple spreadsheets in it)

It has the same 'stall' whether I point Feed Import to pull from a server folder location automatically, or if I just manually browse and import the file directly.

I'm not getting any log messages in Reports --> Recent Log Messages or Reports -->Feeds Log.

In settings, I added $conf['feeds_process_limit'] = 2000 based on feedback from a somewhat similar problem in feeds. But that didn't help.

It appears that in these instances, it's bringing in all the data from the first spreadsheet of the first workbook, but then nothing else. I've got the spreadsheets set to UTF-8 encoding (which is important since some of the words are in German and have umlauts).

Any advice on what else I could try, or a secret backdoor log I could review that might shed light on why it's hanging?