There are stumbling-blocks to ordination, and stumbling-blocks to realisation of the Dhamma. Which is it that you want to know?
The ordination procedure is given in the Mahāvagga, where the candidate is questioned about his suitability: “Are you a human being? Are you a man? Are you a free man (not a slave)? Are you free from debt? Are you free from military service? Have your parents permitted your going-forth? Are you fully 20 years of age (from the time of conception)? Do you have a full set or robs and an almsbowl?
The other stumbling blocks are things like concealing an offence, having insulted a noble one, etc. I don't know where to find a complete list.
• • • • (Upasampadā: 24th June, 1979)