Contact sheets have a "maximum" black inconsinstencies throughout the same 35mm roll:

When making a contact sheet, I aim for maximum black using a strip of negative, but often when exposing the whole roll, the exposure time to get the rebate black is not the same for all 6 strips of cut negative.

The ones that are done using my test strip are matching, but others are more exposed or less exposed.

In these cases, do you dodge and burn the entire strip sections that are not equally exposed?

What is the cause of these exposure inconstancies?

Maybe uneven light source, contact sheet frame not placed centrally under the light source, not developing the paper to completion, or other things that can lead to inconsistency are too short an exposure time. It is strange if it's only the black rebate that is inconsistent and not the images themselves.

@Rorystreet91 is the problem:
1: That the film base is black on some strips of the full contact sheet, but not on other film strips on the same contact sheet?
2: That the exposure time you determined using one or two strips of film does not match the exposure time you need to get the same level of black for the film base for the full contact sheet?
It's not entirely clear which of these is your issue, but I assume/believe it's #2. Assuming that, three potential issues come to mind. The first is that something has changed in the setup between the test strip exposures and the full sheet exposure; e.g. a different contact printing frame or different distance between lens/light source and print. The second is that consecutive, but separate exposures (e.g. 3 exposures of 1 second each) do not necessarily yield the same overall exposure as a single exposure of the same total duration (e.g. 1 exposure of 3 seconds). A third possible factor is simply density differences in the film itself; e.g. due to trying to contact print strips from separate rolls that were developed separately and that might even have been different types of film.

It would help greatly if you could describe in detail what exactly you are doing to get more clarity on the nature of the problem.
Also, pictures would help a lot.