[Haskell] repa not running in parallel?

Hi, I'm new to haskell and I tried to reproduce the perfomance values from the paper "Regular, Shape-polymorphic, Parallel Arrays in Haskell". I modified the repa-mmult example from the repa-example package to use the mmultP from Data.Array.Repa.Algorithms.Matrix. Then I compile it with "ghc -threaded -fglasgow-exts -O Main.hs" but running it with two large matrices : "./Main -random 4096 4096 -random 4096 4096" I can see that only one of my 4 cpus is utilized. What am I doing wrong? Note: The unmodified repa-example also does not run in parallel. Thx, Alex