@Pashalis I use machines that isolate each movement. I'm don't think it's the specific brand McGuff recommended.
So those machines likely don't have the cams in them that the Nautilus/MedX have for the specific muscle profiles. How much weight do you use? Are you making sure to not use too much and not too little, aka. the 75% rule he mentions in the book? When your max power on a specific part, is, lets say 100kg (meaning one repetition possible at most, no matter how fast), then the weight should be around 75kg. This Max should theoretically and hopefully increase while the ratio weight increases as well, in the course of the months.
Usually you shouldn't be able to do more then 5 or 6 repetitions with the optimal weight. If you can do easily that number or more, then the weight is probably too light. If you can do only one or two, the weight is probably too much. Both is not optimal according to this program and can have negative consequences, also in terms of improvement.
Are you making sure to keep a good workout journal in which you can check parameters regularly? What I included in my sheet for example is the sum total time of the whole workout including the breaks and exercises (for that I start the time right before the first rep on the first machine and stop right after the last rep in the last machine ) which Guff recommends. On a second clock I count the time for each exercise and then at the end subtract that amount from the total time and divide it through the number of exercises. That is what Guff recommends as well. In that way, you can check if your time between the exercises (the time you walk to the next machine and start the next session) stays the same in each week. He recommends that this time should be between 1 Minute and at most 30 seconds. I'm working to get there, my time in between is at around 85 seconds in the moment, since I still loose to much time in setting the weights and measurements up. This should get better with practise.
Lets say you only have 30 seconds between one exercise and the next, what you will notice is that you can do much less on the second exercise, if you compare it to over 1 minute for example. In other words, as Guff explains, certain muscle fibers recover pretty quickly and when you get over a certain point in time, the next exercise will therefore be much easier. Or in the reverse if the time is very short in between you will be able to do much less.
A couple of ideas why you might not be improving.