About the load on UEL with C3D8 element

Hello,

I have written a C3D8 element using UEL in abaqus. The nodes of the element is like this:

*Node
      1,           0.,           0.,           0.
      2,           1.,           0.,           0.
      3,           1.,           1.,           0.
      4,           0.,           1.,           0.
      5,           0.,           0.,           1.
      6,           1.,           0.,           1.
      7,           1.,           1.,           1.
      8,           0.,           1.,           1.

The boundary condition is just to constrain the rigid body moment for the elemnt. See following,

*Boundary
1,pinned
2,2,2
5,2,2
6,2,2
4,1,1
5,1,1
8,1,1
2,3,3
3,3,3
4,3,3

When I load on face, such as give same displacement for the four nodes on the same face, it works very well. See following boundary.

*Boundary
5, 3, 3, 0.1
6, 3, 3, 0.1
7, 3, 3, 0.1
8, 3, 3, 0.1

 
So, I think the RHS and AMATRX are correct.

But, when I load on just one node, the result appear to be diverging. Such as

*Boundary
6, 3, 3, 0.1

 
Could anybody give me some suggestions about this problem? Thanks!