tf exception map to base_link

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Hi all

We have a robot with 2 machines (1 quad core, 1 dual, both SSD) that makes heavy use of tf.

With everything running, we have about 80 nodes/nodelets. The machines are certainly being taxed, but we have found that recently tf lookups are simply not working. Here's a typical exception, though it's not the only type we've seen:

Caught Transform Exception: Unable to lookup transform, cache is empty, when looking up transform from frame [/base_link] to frame [/map]

I am particularly confused about how the cache could be empty, unless the machine is hung so badly that absolutely no /tf data is being received.

Also, if we are logged in remotely to a machine and call rosrun tf tf_echo /map /base_link, the echo works as expected.

In this particular example, I am calling the tf lookup as follows:

  try{
    ros::spinOnce();
    tf::StampedTransform fixed_to_base;
    listener_.waitForTransform(fixed_frame_,base_frame_,msg->header.stamp,ros::Duration(30.0));
    listener_.lookupTransform(fixed_frame_,base_frame_,msg->header.stamp,fixed_to_base);

This is happening with calls in multiple processes, both C++ and python, so is there anything we should be doing differently?