Jenkins sometime fails to auto-promote job -

i have big net of jobs. this: start job: a, job promote: z, requires: e, f, g

 a------- |  \     | b  c     d |   |   / \ e   f  g   h  |   |  |   |   j  k   l ============ > if(e,f,g) promote z 

the question is: why work? , sometimes( seems on bigger jenkins load) have manually promote z?

are giving enough time execute? promotions take executor run. if server under load, , there no available executors, queue promotion. promotion star appears @ time of triggering (auto or not) of promotion.