Not all pictures/videos migrated to Photos from iPhoto

Product specs: Macbook (13-inch, late 2009) running OS X 10.11.1 with 4GB RAM and a 500 GB SSD.

About a month ago I finally migrated my iPhoto library to the new Photos app. I have over 34k "items", according to Photos. When I first tried to migrate it froze, and I had to force quit. Then I tried to migrate again and it seemed to work okay. I'm impressed with the speed of the new app compared to iPhoto & Aperture.

Fast forward to this week, when I started to create an album of the best pics of the past year for a Christmas gift book. As I was going through the pics and videos from the past year I noticed many were missing from some key events - a wedding, a family trip, and other highlights. I soon figured out that that no iPhoto events after May of this year were copied over, and some events (and parts of events) from earlier this year were missing, too.

I believe nothing is really lost - it looks like the missing pics and videos are still available in iPhoto / Aperture, and I use Time Machine. But I really want everything from the old apps imported into the new app. I think I have a ton of tedious work ahead of me - going back and forth between the old and new apps to see what's missing. But given the size of my library (which keeps growing), this may take an extremely long time.

Does anyone know why this happened, and what I can do to fix it. Its tough not knowing the full extent of what might not have made it over. Any suggestions? I held off on migrating my library but figured by the time 10.11.1 came out it would be safe to do so. Maybe I should have waited?

Please advise!!

Thanks!