If I read correct, you do 4 days doing 2 body parts each day and then train your whole body on two days and take one day off?
If that is correct, it doesn't sound like enough rest to me. 60 minutes is more than enough to get a push, pull or leg day in if you don't take 5 minute breaks between sets.
Recently I started reducing my time between sets and the weight I lift went down a bit but it is coming back up - just a change for the body to get used to.
I can only train two days in a row and then I need a day off to recuperate. I don't understand how people work out 6 days a week unless they don't really tax themselves. That was not a slight against you, maybe if I was younger and on a bunch of gear I could but after two days of working out hard, my body is telling me it needs to rest.
I love the PPL system and I and go two days on and one off. I tried three days on and 1 off and I wasn't getting enough rest.
If it is working for you and you are growing / getting stronger keep doing what your doing. If you aren't try something new. I am going to stick with PPL until I plateau which I haven't yet.
Good luck!