Hi Prasad,
if all the config is correct, as you say, then the error must be due to some superior power and there's nothing we can do to help you 
However, if you want someone to provide a sound idea (rather than wild guesses) as to where the issue comes from, please share the relevant config with us.
it is also not clear to me: are you saying that user did see all the subtypes until a few days ago and then all of a sudden he didn't see them anymore? Please confirm, whether I assume this correctly or not.
I will still throw one wild guess into the arena: are you 100% sure the user didn't accidentally create a personal value list in the dropdown? Of course he'll say "no" probably not even knowing what it is, but sit down with him,when the incomplete dropdown list is open and click the global symbol at the bottom. I know, it's a cheap answer,but it still happens soooo often :-)
kind regards
Sven