There are numerous variables that affect curing time: How many coats you put down, how thick the coats are, how long you wait between coats, humidity, temperature, etc. So yes, from one time to another you can get different results.
In any case 3-7 days is not long enough for a good cure. The minimum I ever do is 2 weeks before waxing, cleaning or playing and longer is definitely better, and safer.
If you go to a good paint store they will tell you the cure time for acrylic/enamel paint is 3-4 weeks.
Although I generally use an orbital buffer I would never use a mechanical buffer for the first waxing after painting. Not saying you did, just being clear on this point because I've seen it done.
As mentioned previously surface prep is also very important. I see a lot of people clean bare wood, which will soak up the cleaner like a sponge, and then not give the wood enough time to dry before painting.
While you should put down a clear coat, the same thing will happen with it if not cured long enough. The only difference is you probably won't notice it.