Finite Element Convergence Analysis

qwuasi

1. The problem statement, all variables and given/known data
I have a designed a Finite Element code for a Poisson problem using
Bilinear Element.
- [tex]\Delta[/tex]U= 2[tex]\pi^{2}Sin(\pi X)Sin(\pi Y)[/tex]
U = 0
The exact solution is given by : UEX = [tex]Sin(\pi X)Sin(\pi Y)[/tex].

2. The attempt at a solution

On convergence analysis:Theoretically, if one increases the Mesh Size by
powers of 2,i.e N =2,4,8,16,32,etc. the error should decrease by 1/4.

Although,i have a solution,my error diverges instead of converging.What
accounts for this?Any direction will be appreciated.