In many ways it just takes time.
You will need time to hone your gallery building skills before some TGPs will think your galleries are good enough for partner accounts.
Generally speaking I think TGP owners like to see that you've been around for a while and are serious about posting and have a good reputation of some sort.
Some TGPs won't take new accounts at all because they have enough trusted submitters to last them for years.
It's nothing to be discouraged about though.
I've been accepted sometimes 6 months after applying to some TGPs. Mind you that was because of their backlog not because of my galleries
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