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SO if you changed the dates to start in 1835 or before, they should be there. I noticed that one of my parties that has a start date of 1914 does not appear at the start of the 1914 campaign, so it might appear sometime during the year not the beginning.
Another thing to check is that you have a multi-party system... perhaps if you have a two party system it only shows 2 parties, and if no elections, then just one party...
It's possible for a country file for a scenario to contain a line eliminating certain parties from existence regardless of the starting date for the parties. Though this is rather rare, I think.