Is this a gate that was only ever designed to handle a 737/DC-9/MD-80. The A320 sits higher off the ground than the above, and has different dimensions and access points meaning it may just not fit the gate.
When Sabena introduced their A319/320/321s they had to modify their procedures (but luckily not their gates) because they had problems like the fuel points on the tarmac no longer linedup with the wings when an Airbus was parked.
There are often restrictions on use of gates. Southwest gates were only designed to handle 737-2/3/500s, the 737NG has larger wings and at some stations they can only handle a few at a time. Also, it depends on what is parked on the next stands, for example a gate may be able to accomodate an A320 if a 737 is at the next gate, but not if a 757 is parked alongside etc...
Another possibility is that the gate just hasn't been approved yet for use with the A320. To use a gate with a new type you need new stop markings painting on the apron and the docking guidance system may need to be modified.