It wasn't so much that the Japanese were 'completely ignored', and more that they basically didn't involve themselves in issues that did not particularly concern them in any real way.

Japan's main concerns for the Peace Conference were trying to secure former German colonies in the Pacific, and especially the German concession in Shangtung. Beyond these specific issues, the delegation really didn't involve themselves in discussions in any real sense.

The one exception was the attempt to insert the so-called 'racial equality' clause into the charter of the League of Nations. This was really aimed against anti-Japanese immigration laws enacted in the western states of the US, Canada, Australia and New Zealand.

This was actually largely accepted among the Allied delegations, apart from the vociferous (and unfortunate) opposition from the Australian and New Zealand delegations. The French had no particular issue with this, as did other European delegations. The British were ambivalent, but opposed it on behalf of the Dominions, and Wilson was concerned about losing votes and support in western states if he supported it.

The Japanese tried to force the issue by bringing it to a vote on 11 April 1919 - where it actually gained a majority (11 out of 17). However, Wilson argued that since there was strong opposition, this should require a unanimous vote, they should put it aside and deal with it later - which never happened. The failure of the clause did provoke some resentment within Japanese government and society, which was a part of rising anti-Western sentiment in the 1920s.

Source: Paris 1919: Six Months That Changed the World, Margaret MacMillan, (Random House, 2003)