I've seen banks reject applications for things as simple as an outdated credit report or because your profile doesn't fit the specific product you requested, and they never just mention it like that! What the others aren't making clear is that even if you ask for written documentation, the bank can take weeks or even deny access to "internal criteria" claiming trade secret, so before you get caught up in that bureaucracy, call the branch directly and ask if there was some technical factor like a low credit score, debt level, or simply that the amount you requested doesn't qualify for your profile.
If that still doesn't give you a clear answer, then send a formal complaint in writing and consider contacting your country's financial superintendent because banks have to explain something more than "internal criteria" - that's too vague!