is_user_admin() only tells you if the player is an admin (if they have any flags other than the "z" flag they will be considered an admin by this function; see amxmisc.inc).
If you want to check the flags for a person, you need to use get_user_flags() to get the integer that represents all of the flags that a player has and then use the bit-wise AND to determine if they have a particular flag.
So, to check if they have the reservation flag, you do this:
Code:
if( get_user_flags(i) & ADMIN_RESERVATION )
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