Booking.com complaint: Partial refund rather than full refund

I called Booking.com to make a reservation at the Marquis de Lafayette Hotel. Midway through the phone call the call was dropped. I called back immediately and explained that I was on the phone with a representative and the call was dropped. I was booking 2 rooms and wasn’t sure if both rooms were booked before the call dropped. The new booking agent explained that he could see that one room was booked but not the other and that he could help me make the second reservation, which he did.
When I was done with the call, I checked my visa card and could see that the second room was booked twice. The total amount was 1187.63. I called immediately and explained the situation to a new agent and they said they would issue a refund, which they did. However the refund was only for $866.35.
I am still owed $321.28. I am not sure why the refund was not for the total amount of $1187.63, but I believe I am still owed the balance of $321.28.
I have made several calls to booking.com and they refuse to issue the balance due to me, with no explanation why. If the balance was on taxes and fees it doesn’t make sense. Why would I have to pay taxes and fee on an amount that was refunded to me!!