Not more than 5 notes?

Hi

At the moment I have a setup with 1 keyboard en one pedal board for the base.
Wen I play a base note (on one of the pedals) and play 4 notes at the same time on the keyboard all sounds fine.
But wihen I press 5 notes at the same time on the keyboard instead of 4 the base note stops playing. But the base note continues playing again when I release the 5th note.
And when I play 5 notes arpeggio on the keyboard (not together at the same time, but slightly after eachother but holdding the notes instead of releasing the notes) all six notes (that is including the base note) are audible and perfect.

Is there a setting to solve thius problem?

Thanks

Koen