Why were the warlords a problem for the Zhou?

The answer is:

The warlords were a problem for the Zhou dynasty because the Zhou dynasty was never an entirely unified realm and the local warlords became less identified with the Zhou king and more with their allocated territories.

Explanation:

The Zhou court extended its power by granting authority to members of the royal family and in some cases to favoured local warlords, who established confined forts supported by garrison troops. In some cases, local warlords were accepted as Zhou supporters. The warlords became a problem when they challenged the Zhou order and weren't quickly dealt with by the army. The ruling class was mainly unified by kinship ties. Family relations were strenghthened by arranged marriages where no kinship links actually existed. In this way, the local lords were expected to accept the authority of the king as the head of the Dynasty.

As time went on, the kinship ties were no longer there and the local warlords became less identified with the Zhou king and more with their allocated territories. This tendency was very strong in larger peripheral states. Regional leaders started to ignore their duties to the Zhou court and also started fighting among themselves by the 9th century BCE.