Working out "Can't prove source of funds?" means knowing which question is being asked. Money goes in with almost no questions and comes out with several, and that asymmetry catches most people at their first withdrawal. A casino is obliged to know who it is paying, where that person lives and where the money being staked came from, and it usually asks at the moment it is about to pay rather than when the account opened. Knowing which three checks exist, and what satisfies each, turns a stalled payout into an afternoon’s errand.
The order matters less than the timing. Identity and address are often asked once and never again; source of funds arrives later, usually when the totals passing through an account cross a threshold the operator does not publish. A payout that has sat pending for days while nothing was requested is worth a question to support, because a check waiting on a document nobody asked for is the commonest cause of a stall.
An answer that will not come has consequences worth knowing in advance. An account that cannot satisfy a source of funds request is usually frozen rather than closed, with the balance held until it can, and the deposits already made are normally returnable to their origin. That is the reason to deposit from an account in the same name from the start: every check further down the line reads more easily when the paper trail begins in the right place.
Source of funds is the third and the one people find intrusive. It asks where the money being gambled came from, and the answer is a document rather than a sentence: payslips, a tax return, a contract of sale, a statement showing the deposit arriving from a named employer. A salary is a perfectly ordinary answer and so is a pension, a redundancy payment or a house sale. What fails is an answer with no paper behind it, because plausibility is not what the check is asking about.
Salary, pension and a house sale as ordinary answers
Pending payouts where nothing was requested
Proof of address dated within three months
A frozen account, and what it means for the balance
Online banking screenshots, and what they must show