I have tried several times to complete the tutorial flight from EGKK to EHAM.Everything is set up exactly right in the CDU (FMC).On the descent profile I should be at or above FL055A at the NIRSI intersection.For some reason, every time I approach FL060, the aircraft levels off.In the altitude on MCP I have set FL020 (2,000).There really does not seem to be any reason for it not to continue it's descent profile.After passing NIRSI, I get an error on the CDU that I am unable to maintain or keep the descent profile and of course I overshoot the runway.Any help would be great.The speed and altitude restrictions programmed into the MCP are exactly as they should be.Thanks for any help.Phil