To Erin:
I think Fred's hit the nail on the head: your loaves
have not risen enough before they go into the oven.
There is a certain amount of oven-spring with most
loaves, so factor that into your timing. I go by feel
and look, not by the amount of time that goes by since
I formed the loaf. As a general rule, the size of my
loaves almost doubles, not quite, when they're ready
to go into the oven. I think Fred's solution is very
sensible. And I think you should keep on slashing: it
releases a lot of tension--in you and in your loaves.
My favorite implement is a very sharp, serrated knife.
Happy Baking, Erin!
Terry