There are a variety of reasons for the panel not fitting the opening easily. These can range from poor measurements through parallelogram, trapezoidal openings or irregular perimeters of the openings.
In the cases of irregular openings, you can trim the edge cames if you have made them 12mm (1/2”) or more wide. The quickest way of trimming cames to fit the opening is to use a rasp or “surform” tool. The open nature of the teeth, allows the lead to fall away. It is much quicker than using a lead knife, and it puts less pressure on the panel.