You need to start making test strips. Try this:
* Set all filters to 0 (no magenta or yellow; cyan is never used anyway)
* Set aperture to f/5.6
* Cut a piece of paper approx. 10x15cm (size is not critical at all)
* Tape the piece of paper inside the image area on your baseboard/easel. It's nice if it covers light and dark areas of the image at the same time.
* Expose for 2 seconds
* Place a piece of carboard on the paper so that it covers a 1 cm narrow band on the edge of the strip.
* Expose for another 2 seconds
* Move the piece of cardboard so that it covers a 2cm band of the strip (so basically move it a centimeter onward)
* Expose for 4 seconds
* Move cardboard again 1 cm
* Expose 8 seconds
* Move cardboard 1 cm
* Expose 16 seconds
* Move cardboard 1 cm
* Expose 32 seconds
You now have a piece of paper that has adjacent bands exposed respectively for 2 seconds, 4, 8, 16, 32 and 64 (i.e. 1 stop increments starting at 2 seconds).
* Place piece of paper in developer. Develop for the amount of time indicated on the developer package. This is usually between 1 and 2 minutes. the development time should be chosen in such a way that no change in the image occurs anymore when development is done. THIS IS IMPORTANT. Don't stop development when the image is still changing. Paper needs to be developed 'to completion', otherwise there will be problems with contrast and maximum density (weak blacks).
* Stop, fix, wash briefly and dry the paper.
* Evaluate results.
> If none of the bands contain a pure, deep black, exposure was insufficient. Do the test again with a larger aperture or longer exposure times. (However, with the settings above, you should almost certainly have a good black somewhere on the test strip).
> If the 2-second band already has a pure black, choose a smaller aperture (e.g. f/11) and redo the test.
> If contrast is too low in all bands, increase magenta filter. Don't increase it just a little; add 30 or 50 units so that the difference is significant. Re-do the test.
> If contrast is too low in all bands, decrease magenta (if it's not 0) or increase yellow (if magenta is already 0).
If this systematic approach to making a test strip still gives too low contrast, and you are 100% sure that the developer is diluted in the right way and the paper is fresh, then the negative is too low in contrast. If you are 100% certain that the film was developed correctly, the only option left is that you underexposed the film significantly. Try a different negative that looks more contrasty if you hold it up to the light. If none is available, shoot some new film, carefully metering the scene, develop it (or send it out to have it developed) and go back to making test strips.