TurboCAD orthographic and parallel dimension problem: where am I going wrong?

I cannot properly dimension anything from a datum end.

This was highlighted on the side elevation of a two-throw crankshaft nearly a foot long, with various intermediate and end diameters as well as the webs. Geometrically it is a chain of joined rectangles of various sizes.

I thought I had to Snap the dimensions to the Vertex and Intersection points, but an error message tells me "No snap points in snap aperture", and the cursor won't release the dimension.

I find I have to turn the Snaps off to set the dimension down, but that leads to inaccurate lengths by having to place the cursor entirely by eye.

Circles and arcs are not a problem: the dimensions work on them, so it's specific to straight-line features.

Anyone know where I'm going wrong, please?

Is it because the rectangles are all joined but are of different heights? I can't seen it would be that, because that would dimension the resulting diagonals.

[Incidentally, I would have asked on TC's own Users' Forum but its log-in routine from my computer was always fragile, has now broken; and so far, IMSI has been unable to re-connect it for me.]

Thank you Tom.

Sorry about the slow response.

Since then I've regained access to its own users' forum.