missions wont let me add 4th ambush?

Oky, im using a CoT map, and it states there are 10 allowable ambushes. I need to add another to something for story purposes and its not letting me. any idea why i cant? the ambush bar is greyed out at 3.

what else have you added to the mission ?
when you select the map it gives you the options if you combine these option you may take away from others such as clickables floor these take away from spawn groups and placeables as well as ambushes.. set your ambush on a clickable or boss one at a time. if you select 1/4 3/4 and so on the may spawn from different locations on the map..so frount middle or end...having seperate ambushes per clickie or boss is beeter and creates a well founded story.

Exactly what dr said. Usually ambushes are combined with other mission objectives, like bosses or something. Ull probably have to cut one thing to add the ambush.