Hi Starbage1,
which era are you flying? There is a different campaign xml for each era. The first thing to check is that the start and end dates for each aircraft in the techtree provide an unbroken availability of a bomber and a fighter of each participating nation, over the whole period of the campaign.
You can also correct any errors in the Techtree start dates in the campaign xml at the same time. For example, if a Spitifire 1a has a start date in it's aircraft xdp of June 1940, then the start date in the campaign xml techtree section should not be May 1940.
HTH,
