Hi,
Plate element should me be modeled using some rules. A plate element’s aspect ratio should be as near to 1:1 as possible.
Aspect ratio is defined as the ratio of the length of the shortest side to the longest side of the element. Aspect ratios in excess of 4:1 should be avoided. In other words, a triangular (3-noded) plate element should be shaped as near to an equilateral triangle as possible, and a quadrilateral (4-noded) element should be shaped as near to a square as possible.
You may consider breaking the plates into smaller pieces.