Allergies and an economizer
A MERV 13 filter should not let much if any pollen get through. If you still have pollen getting past, it is likely leaking around the filter. 

I would check that the filter is secured tightly in the HVAC system and that there is no place for air to bypass the filter.

To increase filtration of existing plain filters, you can also spray it with a tackifier. The tackifier will catch particles small enough to bounce off the filter fibers but pass between the fibers.