板材振动分析的边界条件问题

三块拼接起来的板的竖向振动，四边简支。在模态分析时，若每边的边界条件均耦合到一个点上，会出现这样的错误：12 nodes are used more than once as a slave node in *TIE keyword. These constraints are not removed either due to the presence of *CONSTRAINT CONTROLS,NO CHANGES keyword or because removing the constraints might affect the model. The nodes have been identified in node set ErrNodeOverconTieSlave.我觉得原因是同一个板角的node不能同时耦合到两个面上。
当我把耦合去掉，边界条件直接加到面上，就会出现这样的错误：
Three factorizations in a row failed. Check the model. It is possible that the model contains the kinematic coupling definition set up in a way that a degree of freedom has neither mass nor stiffness。
想了很长时间，不知道什么原因，希望大家帮帮忙，先谢谢大家了！