Why cannot Mathcad Perform the Duhamel Integral?

Hi, guys,

As below shows, when I tried to perform a Duhamel Integral in Mathcad,

If I manually copy and paste P1(tao) into D2(t), Mathcad will integrate successfully, as shown in the 2nd D2(t)

However, when I used P1(tao) in the integral, the integration is not successful, as shown in the 1st D2(t), where the result expression has tao inside that is not supposed to be there.

Could someone help me explain why?

Thank you!

Best

Shawn