Are delays and 'being stuck' normal in PhDs, and how much should I work to compensate?
What you describe seems pretty normal, except for the disappointed supervisors. But maybe your work habits are inefficient. It seems like you may be stressing time in the seat too much. If you don't take regular breaks, get enough sleep, and tend to your health, then your mind won't work efficiently. You will get stuck and pounding harder isn't the way to think deeper. Take a break regularly, especially when you get stuck. It doesn't need to be long or elaborate, but a short walk, run, or bike ride, can restore both your body and mind. 

Your mind doesn't just work when you try to force it to. It actually works all the time, even when you are sleeping. Sometimes the break is what it takes for your mind to stop focusing on minutia and start to integrate what you have been looking at. 

But, if you are really doing research, and not just going through the motions, then you can't schedule results. They come when they will. 

“If we knew what it was we were doing, it would not be called research, would it?” ~~Albert Einstein

Another trick is to talk to people about what you are doing. They need to have some knowledge, but if you just try to explain it you will, perhaps, come to a better understanding yourself. Some people even "talk" to a Rubber Ducky that they keep on their desk. Tell it to the Duck. But people can give you feedback. 

The "dead ends" you explored may also actually be valuable, later if not now. They might actually do nothing more than provide a break from trying to push forward. And those breaks are essential. 

If you do too many push-ups your muscles get saturated with lactic acid (I think I got that right) and weaken. Something similar happens with your brain. Work hard, but take breaks. For me, random breaks, rather than a schedule, works best. When I'm not being productive, do something else for a while. 

But if your breaks take you away from your desk as they should, you can also carry a few note cards, including blank ones. You can use these to remind yourself of partially formed ideas and capture new ones if they occur during exercise. Don't force yourself to work on a break, but have them ready to capture any serendipitous thoughts.