Step climb

Hi!

I have problem with step climb, when I load the route into the FMS everything is OK, but when I’m getting closer to the step climb it keeps getting further away on the ND. It can be that I have 200 Nm left to the step climb and 3 minutes later it shows 450 Nm.
Another thing that I saw was that Simbriefs flightplan does not match when I load it into the FMS, it shows different altitudes, am I doing something wrong?