The markers probably have EXACTLY the same coordinats so they stack on top of each other hence it appears to you as if there would be only one. You have to give each marker distinctive coordinates to see each marker.
Now I experience the same problem. Despite having no limit and pagination to "All" only 11 markers of the 300 locations are displayed on the map. Is Google restricting the API or is this a SEBLOD issue?
The debug says that all locations were fetched from the db, but still only 11 show up. They all have different lat lng and all have a marker icon.
When I switch pagination to 10 and go through the pages all locations show up 10 by 10. But I need all locations on one single map.