Thanks to the Enlighten feature that allows the user to review the PV module energy statistics graphically, I was alerted that my solar attic fan was causing a shading issue on one of the modules for more than an hour in the late morning.
The solution was obvious: I just had to climb up and lower the module on the attic fan to reduce the shading. Now the vent module is oriented toward the sun in the late afternoon rather than midday, but that is when the heat is highest in the attic, anyway.
The Enlighten software shows the shaded module producing only 15 watts.
Adjusting the small PV module on the attic fan helped reduce shading on an adjacent module.