Could be the film - it's very nicely graded with smooth transitions IMO, but certainly not very contrasty. However, this is actually quite desirable because it's reasonably easy to change (or correct) contrast in post because of this - scan to TIFF (no compression), then do post using curves and black/white point correction before exporting to JPEG; the contrast slider and/or a zone system tool might help as well - darktable offers all of these; but it's also simple enough to do from within Polarr ... I took the freedom of downloading your JPEG, and even with that (i.e. from non-optimal data, but a nicely balanced image), Polarr made it almost trivial to enhance contrast without ruining the image (I deleted the result - it's your image; but if you would like me to, I can easily redo it).
Oh please go ahead Matt, I would love to see what Polarr produced. I actually scan linear and get a "raw" negative which I then invert in LR and take it from there. The thing is since I am so new to all this, I am not really sure what HP5Plus400 is supposed to look like.