Why is simulation taking so long?

When using Simulink, usually when I run the model I can view the results more or less instantly, however sometimes I hit run and it is far slower. I can look at the scope and see the data being slowly filled in. Can anyone tell me why this is? I don't think I am doing anything different from when the model runs instantly.

[Jon]

How can you expect any help when you provide no information about your model?