Can't re-import a majority of products in CSV file I have been working on a development site for a long time, so there have been many instances where I have imported, deleted, re-imported, re-imported with tampering, etc. There seems to be a large chunk of products from the CSV file that keep getting skipped during the import, and I can't figure out where there is a record of them in Drupal that could cause them to be "flagged" as already imported.

One possible source of the problem may involve the CSV files. The client is exporting a CSV file of all products on a weekly basis from an incredibly old computer system. The filename is always the same, and I have set up the feed with the File Upload settings set to "Supply path to file or directory directly."

I've used the Commerce Reset module, cleared caches, rebuilt the feeds importers with different machine names - everything I could possibly think of - yet it skips several hundred products that I know I have imported before. Is there something I'm missing, possibly regarding GUIDs, UUIDs, product_id/entity id?

EDIT: 

I should also have mentioned that I am using two feeds to import products - one to add the prices, and one for everything else (it's part of the client's DOS-based system, so no flexibility there). The current process involves running a feed to create the product (add the SKU, title, keywords), then running the price feed, which is set to update only. 

Last night, I simply switched the settings on the price feed (add new products, overwrite existing, skip hash check, delete non-existing - the same as the other feed) and ran it. I was able to import all of the products. However, I decided to run the other feed to get the descriptions and keywords, and it would only update the products that it had last imported when it was set to create/overwrite/delete. What is happening here?
There is a bug report about Importing Large CSV document (downloaded and processed in the background). 

Also, take a look at Feeds large CSV import.

So if your CSV files are too big, parhaps cutting them down into multiple smaller CSV's will likely fix your problem OR You could try using the Drush Feeds sandbox as mentioned in the first comment.