The OP had the most important part of the answer in his question. The most basic answer is when the engine is unable to advance from the point of starter assist to idle as the "cores" fail to accelerate each other.
Could be a result of bleed loss, a rich or lean mixture that will not atomize well (rich could be early starter cutout) (lean could be a fuel metering issue). There are also variable stators in the compression section that could be adjusted incorrectly or lagging. Probably a combination of several factors high and hot airfields not helping much.