Quote Originally Posted by swittmann View Post
In those cases, the negatives were underexposed as well, so I assume that there is a correlation. What do you think? Is this possible?
Perhaps your negatives are not underexposed but underdeveloped. It can be hard to spot the difference. If there is sufficient detail in the shadows then your negatives are probably only underdeveloped.

In case there is underdevelopment this may have been caused by insufficient agitation or by too small a ratio of agitation time and total development time. (There are other possible causes of underdevelopment, but they shouldn't be relevant in this context.) Too little agitation in turn may give rise to uneven development. So yes, there could be a causal relationship between uneven development and underdevelopment mistaken for underexposure.

Quote Originally Posted by swittmann View Post
I am using both a Paterson tank (500 ml) and Jobo 1520, and my developer is Spur HRX-III, which requires 2 initial inversions, then 1 inversion every 30 seconds, so not much movement here.
Two initial inversions is a bit little. Try at least 30 seconds of initial agitation. Proper initial agitation is crucial in obtaining evenly developed negatives.

Also, if you're using the smallest size Paterson tank, try to get one size bigger (the one that takes two 135 films). Agitate such that all developer is allowed to clear the film, in other words invert fully and not too short. This allows the agitation to 'look' the same to any random area of the film.

Quote Originally Posted by swittmann View Post
Film is Acros or Tri-X, sometimes/seldom Delta 400.
The film and the camera seem unlikely culprits.

Good luck.