Just pulling my hair out in frustration with continually being rejected for caption reasons this is the latest  -"Three men wearing Elizabethan costumes riding gray horses at Audley End House Essex"  i cant think whays wrong with it.

it needs to not only be complete, but to follow the iStock required format regarding the placement of the date and the place in the caption.  I don't remember the format because I stopped uploading editorial there for other reasons, but if you look at the examples and follow those, it's not too hard.  Deep Meta also helps because it includes an example as you submit.

I use to sometimes get rejections for captions, until I learned to make it as long as possible. Not a single rejection since. I noticed your caption is only 1 sentence. I would try to find more things to say to make it at least 2 sentences, if not 3. Good luck!

I don't know what your picture looks like, so I'll make up some info up as an example.
Date - Location, Three men wearing Elizabethan costumes riding gray horses at Audley End House Essex. The three men are reenacting a famous scene from the 1547 battle of goats. There was a famous disputes over the missing royal goats that the queen loved.

There you go, much longer.