sharepoint permission issue

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SharePoint

i have one site collection which has 4 subsites. no subsite inherits permission from parent. i have one group called "BU". i have given permmision to this group with read access to all the 4 subsites and site collection too.
now my problem is,
when i login with any user from "BU" group, 3 subsites are visible but one is not....and if i give Contribute access to "BU" group then all the 4 sites are visible to user of "BU" group......why this is so??
waiting for rpl..
Thanks in adv...