TLS_PSK and perfect forward secrecy
Assuming you are talking TLS_PSK without DH or RSA (which are an option), if an attacker compromises the PSK and watches the nonces go past in the clear, yes the attacker can easily compute the master secret.

Section 2 of RFC4279 details how the premaster secret is computed. Without DH or RSA, there is no, indeed can be no*, additional entropy added.

The master secret is a function of the premaster secret and two nonces sent in the clear at the beginning of the handshake. So if the handshake is captured, so are the nonces. So all the inputs to compute the master secret are known.

* Any additional entropy that could be added must be shared. TLS_PSK assumes that the PSK is the only shared secret value between the two parties.