The T-brackets for the outside are not the same as the ones that go in the middle.
ALSO, you may need to double check your alignments. It's a bugger to put together, especially that middle part. I leave my intact. Then again, I have a truck big enough to put in a 15-foot i-beam truss. I have 2 10-foot trusses(2 5-footers) and 2 5-foot segments that I use for ground mounting.
Check your alignments and orientation and work one edge at a time.