Very slender members tend to be broken during production and transportation. The BD sets an upper limit on the slenderness ratio of a member. A member passes the slenderness check if the length divided by the radius of gyration is less than 300, or L/r<300. This means longer, thinner members tend to have a higher slenderness (they are more "slender"). Also, solid bars have a higher slenderness than a tube with the same length and cross-sectional area. The load test animation will not be run and the bridge will fail as a whole if any member fails the slenderness check.
There are a few ways to lower the slenderness of a member: decrease member length, increase member thickness, or change the member to a hollow tube. However, it is more beneficial to set cross section type based on the load a member is bearing (read more here).