SharePoint Board view bucket limited to 9 or 10 items

Hi everyone, 

I have recently started using SharePoint list Board view which is perfect for my needs, however my buckets seem to be limited to a maximum of 10 items. When I drag the 10th item into the bucket I get a red circle with a cross appearing at the top of the bucket and the card I tried to move to that bucket is put back where i dragged it from. Screen shot below of the bucket heading and the icon I'm talking about. 

Can anyone tell me what is going on here and how I can fix this? 

Thanks in advance!