I cannot really help you any further without an example. As your problem is defined on a cylinderical mesh, it might be that the linear approximation of the mesh geometry reduced convergence.
A suggestion for you is to try to make a manufactured solution for your PDE on a simple domain, see: Solving PDEs in Python - <br> The FEniCS Tutorial Volume I
for an example for Poisson problems, or Manufactured Solution for Navier Stokes - #2 by dokken for the Navier-Stokes problem with manufactured solution.