In Pazy's book on semigroups he mentions (page 18) that when you have a commuting family of operators $B(t)$, such that $$ \sup \| B(t_1) .. B(t_n) \| \le M $$ for all finite choices $t_1, .. t_n$ then you can renorm the operators into a contraction, by choosing the sup of all such finite sets. So far so good. He further mentions that in general $$ \| B(t)^n \| \le M \qquad \forall t \forall n $$ will not be a sufficient condition in general whilst for resolvents (his case with Hille-Yosida) is does.
Regrettably, he does not give any example of hint. Do you have one, please?