Convergence problems
 

Could some one please explain if the lack of prisms and the use of a pure tet mesh can be a reason for convergence problems in CFD

No. May be accuracy is lost, but shouldn't affect the convergence.

I respectfully disagree. A poor quality mesh can hurt convergence in several ways. Assuming a FV discretization, integration points will be further from the vector connecting neighboring cells, meaning the method becomes more "explicit" and will take more coefficient iterations to converge. Also limiters can be more active, which hurts convergence.

Now I am not saying this in general for tets. It is more mesh quality that can be a problem.

I agree ;)

some updates from CFX forum
 

Here is update from CFX forum

Another good point. Tets are certainly less accurate in boundary layers in part because of greater false diffusion and as we know a more diffusive scheme is easier to converge. Like I said before if the mesh has very bad quality that could cause convergence issues, but most likely it is something else. You should absolutely be able to get convergence without prisms.