Missing Data in Scatter Chart

I created a simple table which I converted into a scatter chart.  The problem is that much of the data that can be seen in the table is not shown in the scatter chart.  i.e. the table shows 17 people with an Activity Score over 80 that also have an Average Test Score.  When I switch it to a scatter chart only one of these is represented (Activity Score 84.4, Average Test Score 93.0).  Where might the other 16 have gone?  Could it be there is too much data in the table (23,748 people)?



Thank you in advance for your help.

Best,

DHB.