If consumers are not pulling messages because the system memory threshold has been reached and producers are still able to add messages, it is not very clear why the consumers are being blocked. A warning when this instance occurs could be helpful in troubleshooting. In one case it was a misconfiguration:

~~~
<systemUsage>
<systemUsage>
<memoryUsage>
<memoryUsage limit="1 mb"/>
</memoryUsage>
<storeUsage>
<storeUsage limit="100 gb"/>
</storeUsage>
<tempUsage>
<tempUsage limit="50 gb"/>
</tempUsage>
</systemUsage>
</systemUsage>
~~~