I have three children under 17 and paid $1,000 plus each in respect of eligible activities yet the online program appears to not be calculating my tax credit. Why?

The fitness credit is limited to $500 per child and you only get the benefit of $75.00 per child.  $500 x 15% = $75

The reason I did not see the credit is that it appears in a different section of the summary since it is a refundable credit vs the Children's Arts Tax Credit.

Glad you see it now.