When the station is very heavy, it becomes extremely difficult having enough mass to reach the desired velocity. This is the same reason why there's a total payload limit for rockets and given a technology there is a maximum theoretical limit of payload to orbit.
This is no different.
Unfortunately, once you build to many thrusters, they become too hungry, because now not only your objective is to reach the asteroid belt, but to reach it in 24 days. It should still be possible, but the solution at convergence might require A LOT of matter.
I should implement a way to reduce the thrust and change the parameters of the trip, but I'm not very good with UI.