As others have said, it is due to a thin file and little payment history. Barclays won't risk credit on you over only three payments history. Anyone might be able to make the first few payments but then what about a year or two from now...how are you still doing? They want to see the track record you build. Again, it's a marathon, not a sprint. Just keep using your cards, don't be late with payments, try to pay in full, and keep the util below 30%.
Cheers!
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