I had something similar happen last month when I went to a new doctor in Auckland where I am visiting for the next few months.
The doctor was a very experienced lapband surgeon. When he had done the fill he had found a level of resistance initially that then suddenly seemed to release and allowed the fill to occur.
The fill was very uncomfortable so I was anxious to remove a small amount from the lapband. When the doctor attempted to insert the needle into the port he once again struck a level of resistance. The doctor explained to me that sometimes small fibres can block the mesh on the port. Heproceeded to flush the port with saline. this was successful and he as then able to gain access to the port to complete the unfill.