I feel your processing times are off.
I agree with Marcus that the development time for your sheet film is too short. It also looks as though you may not have covered the sheet of film completely at once with developer.
Either way, ensure that you have sufficient solution to fully immerse each sheet of film.
Change your dilution to allow longer development times. Start with 10 minutes as a minimum. Don't be in a hurry. This step determines your success. Longer processing times minimize timing variation errors due to immersion, draining, and transfer times.
There are a few different techniques to tray develop sheet film. Some use special soft brushes soaked in developer to "paint" the developer onto the emulsion. Some tape the film to the centre of the bottom of a dry tray and pour the developer onto it. More often, the sheet of film is held by the notched corner in the right hand (so the emulsion faces up) and slid into the tray of developer in one smooth motion.
For accurate repeatable results, sensitometric processing requires fastening the film with tape and rocking the tray determines the agitation. Whether you fasten the film or not, the agitation technique is the same, more or less.
Sensitometric Agitation Technique for Tray Processing Sheet Film
(Practice this technique first with water in a normally lit room until you can do it neatly.)
- Pour the developer into the tray.
- Start the timer.
- Lift the closest edge of the tray about a quarter of an inch and
lower it to the counter.
- Lift the right-hand edge about a quarter of an inch and then lower it
to the counter.
- Lift the closest edge again and lower it to the counter.
- Lift the left-hand edge and lower it to the counter.
- Repeat this rhythmic cycle until the end of the development time.
For the timing, take one second to lift each edge and another second to lower it. Two seconds per side; front, right, front, left, front, right, front, … etc.
At the end of development, replace the developer with an acid stop bath to change the pH of the alkaline developer to stop the development of the latent image. The surface of the emulsion is fully swollen with developer and is easily scratched. Handle it with great care.
Allow 30 seconds to a minute for the stop bath.
Fix the film for twice the time required to clear the film.
NOTE: To establish this time precisely, immerse a piece of unprocessed film in fixer and record how long it takes to completely clear. Double the time it takes to turn clear to get the required fixing time for processing your sheet of film.
Now, you can turn the lights on.
At this point in the process, the emulsion is laden with fixer and silver salts which can destroy your image in time if not removed. The residual chemicals must be washed free of the emulsion which takes time. Wash your film for a minimum of 30 minutes in running water. It doesn't need to be a torrent. It just has to be sufficient to change the water in the tray a couple of times a minute. Take care to keep wash water temperatures as constant and consistent as you did for the rest of the process. Film emulsion is susceptible to thermal shock when wet and can reticulate easily which will ruin the image.
You may or may not need to use a wetting solution after a thorough washing.
Hang up to drip-dry in a dust-free place.
Admire your excellent results
If you want to check the regularity and homogeneity of your processing, fog a sheet of film evenly and carefully process it as described above. Any processing irregularities will be exposed :) as there will be no image to distract you. Film can be fogged by putting a sheet of film face-up on the darkroom counter top and turning on a small indirect light very briefly.