I don't quite understand what was going on here either.
You might have to give more information.
From what you are saying the spread is .25, yet you are only asking for .20? You said Bull Put spread right, so a credit spread? Normally a person would then ask for .25 or higher, not lower - maybe your example was off?
Other things I could think of that could possibly cause confusion -
Did you by any chance mix up the positions, so trying to sell the lower put and buy the higher one, and still ask for a credit? Of course, that would be a debit position.
Also, what exact index or symbol were you using - for example SPY? SPX? Could it have been there were adjusted (non-standard) contracts that you were selecting somehow?
The terminology you use and maybe the person on the phone used of "credit limit" is confusing (sounds like a Credit Card). Better is "limit credit", but again, are you sure the position was setup to where credit makes sense. Also, a person should be able to put in an order for a price far from the current, and not have it be canceled on them - so I have to wonder if something else is wrong????
It does make sense NOT to use the LAST value, as that potentially could be far from the current bid/ask, however, if the options you were trying to trade are active (I would think so for an index), that wouldn't seem too likely.
Just some of my thoughts and guessing.
JJacksET4