Here's a copy of the email I got back:
Thank you for contacting the Your More Value for the Way You Live e-Coupons
help center.
To load coupons to your card, first register or login to your Unilever
account.
Once you login, you are directed to your Unilever coupons page where you
will be able to view your active coupons. To add more coupons you will
need to click "Add Coupons" to view and select new coupons. Click "add to
cart" next to the coupons you would like to select. Once you have selected
all your coupons, click "Load to card" at the top right of the page.
There is a maximum of 50 coupons that may be loaded onto your shopper's
card.
Once coupons have been entered onto your shopper's card, they cannot be
removed. They remain on your card until you redeem them or they expire.

So anyway, since they basically told me what I already knew, just for the fun of it I tried loading them again this evening & it worked. However, I think I might be near my 50 limit on my card, what with Cellfire, Shortcuts, and P&G, so maybe that was the problem. I did start adding them one at a time, so maybe try that!