That's an interesting question Katie,

In the conjugated examples of the lesson , the only one where there's no liaison between the 't' and the 'est' is the 'tu' form and I can't justify why.

You just wouldn't say "Ça t'est_égal" 

There are some liaisons that are compulsory, 

on_a , vous_avez, un_ examen

and some forbidden like after 'et'  

or to sound a 'h aspiré':

des héros, des haricots, en haut.

Maybe a lesson on this would be useful...

Hope this helps!