I'm trying to take the derivative of a function with respect to t but it shows as 0 when it is not. could you please help?
You've been hit by Prime's infamous autolabelling feature.
The t in the d/dt is labelled an automatic (variable):
instead of a real variable:
So change its label to variable and all should be as expected.
I can verify the error in Prime 4 and Prime 5 by opening the original sheet and recalculating it (after deleting all the irrelevant and time consuming regions).
As shown by Luc the workaround is manually labelling the t in the derivative operator as "Variable".
In Prime 6 this bug seems to be fixed - I verified it with both symbolic engines available in P6 (muPad and FreeCAS). So no surprise that its still fixed in P7 😉
I'm using 3.1 though
As already written - this is a bug and this bug was only fixed in Prime 6.
So, unless you intend to update your version (which IMHO would not be worth the money), you will have to resort to some kind of workaround.