With regards to the lack of premierships from Uni's middle/lower grades IMO it's a mixture of what CAT, RH and The Big Dog have been saying above.
The major differences are:
Quite often it is a case of young men playing seasoned men in grades 1-4. For example the UQ 2nd grade back line last week consisted of 5 x 19yo players straight out of colts... playing against a couple of ex premier guys. That makes a pretty big difference. Take the stacked Norths 2nd Grade last year for example... it really was a case of Norths 2nd best XV verse most other teams 4th best. That is the reality of lower grade rugby. It happens year in, year out with a number of clubs.
I'll touch on the point Cat was making. Lets look at if you have your Premier grade team through to your 4th grade team in the finals. Lets, for example, say you have on average 1-2 injuries per team in the weeks leading up to finals/the finals. What you're looking at in 1st grade is 3-6 positional changes, 2nd grade is 4-8 positional changes, so on and so forth. Again this is a reality of that situation. A team such as Logan in 1s, Norths in 2s or Easts in 3s doesn't have that number of respective changes to those specific teams. Their change remains 1-2 with likely subs from grades above who didn't make the finals. This makes a pretty significant difference in Finals.
As we all know, experience is huge in finals. There is a pretty high turn over of players at UQ with a strong Colts program pushing more and more players into Grades each year. Whilst a certain team may have made the finals a number of years in a row, it's never the same or similar team(from about 1st grade down). So you actually lose all that individual experience in that grade from the previous year. A product of the high turnover. In saying that, I think this year will be a little different. A number of players have come back and a fair few have hung around adding that much needed experience. Fingers crossed a few of the boys get the cake in finals this year.... a fair few have earned a win.
None of these are excuses.... just thought I'd share my experience and thoughts on the matter.
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I can think of 3 grades we should have won over the past 2 seasons of which there is no excuse apart from "less than finals" performances. Probably another 3 which we could have won if they had played to their potential during the finals series. But that's Rugby... our loss is another clubs win.