Make sure that there are no conflicts in the settings for the belt direction. For more information, see Best Practices for Modeling Pulley Networks.

Ensure that there is inertia in the system. If there is no inertia, add it by including an Inertia block from the Simscape Rotational Elements library, by adding inertia to a downstream component, or including inertia in one of the pulleys. As needed, specify an initial velocity for the inertia.

Compare the number of pulley pairs to the number of tensioners. If the number of tensioners is not equal to either the number of pulley pairs or the number of pulley pairs less one, add tensioners. For example, if there are five pulley pairs, include at least four tensioners. Use spring and damper blocks to model the tensioners.