Questions about RPIM shapefunction

I am solving a simple solid mechanics problem by meshless method.  use RPIM to  calculate shape function.

when i calculate the derivatives of shape function, i found the derivative is not close to zero at the compute point, when the point is on the boundry of problem domain. and good derivative result can be got if the compute point is in problem domain. 

is there anybody has experience in working with RPIM shape function help me to fix the problem?