Wells Fargo Credit Card Complaint

Wells Fargo Credit card department,

Convenience checks Idaho

I took a Super Check offer from Wells Fargo Bank that offered, I believe, 15 months with no interest. I calculated that $300.00 a month would pay back the check amount by the deadline to insure no interest. When the the card was utilized for other reasons, I paid that amount in full along with the $300.00 and I am still being charged interest. Why??