This often happens when your model is too ideal and there is a sudden transition.
In a hydraulic model, this could for example be that you have a flow without restriction and a sudden pressure drop.
In that case, the Simscape solver needs to go through the "transient Initialization" phase:
and the initial guess he has is too far from the results... consequently it fails to converge.
So I would try to add more realist conditions to your model... depending on the domain, it can mean many different things.
If this does not work, I recommend contacting support with your model.