I'm also having trouble using the HOLIDAY20 promo code on Sonar X3 Producer Upgrade (from X2 Producer). Again, the store seems to think that the promo code has already been used.
Some extra info : I selected the product and entered the promo code before I "signed in" to the store. The discount was correctly applied (£119 down to £104). But as soon as I signed in, the discount was removed, showing the "already applied" message.
I've emailed Cakewalk about the problem, hope they get it sorted so that I can upgrade to over xmas !
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Here's a list of qualifying products:
SONAR X3 upgrades and holiday bundles
Dimension Pro
Rapture
Z3TA+ 2
....