What's going wrong with Form 1116? Done myself, it shows I can take full credit for foreign taxes paid, but TurboTax is only showing a fraction (~15%) of what I paid.

Not all foreign taxes qualify for the credit.  What type of income did you have, and how many foreign taxes did you pay on that income?  Also see:

Filing IRS Form 1116 to Claim the Foreign Tax Credit - TurboTax Tax Tips & Videos (intuit.com)

100% passive income from investments. I was able to deduct 100% of the foreign taxes paid gained from the same investments last year, and with the % calculation of the total amount I can deduct on form 1116 (Line 21), it's still over the amount of foreign taxes I paid, so I should be able to deduct all, but TT comes back with a significantly smaller amount when I walk through their dialogue.