OpenMP problem in katz and pagerank

I have compiled (using GCC version 4.6.3) graph-tool with the option --enable-openmp for the configuration script. The compilation process seemed to run smoothly without reporting any errors or problems.

However, if I use the katz or pagerank centrality index (on various sizes of graphs) only one CPU core is fully engaged at all times. I have found no solution or similar problems on a existing forum. Does 'parallel computation' for these algorithms in graph-tool means open-mp will use multithreating? If not there a way to allow for multithreating using for example the parallel python module?