SharePoint 2013 site permissions issue

Question

Scenario:

• Site collection[ABC] --> Site 1 --> Subsite-1
• Site 1 and Subsite-1 having its own unique permissions. they are not inheriting parent site permissions.

• ABC Visitors is SharePoint group in site collection.

• ABC Visitors has Read Permission over site collection.

• ABC Visitors has Read Permission over Site 1.

• ABC Visitors has Read Permission over Subsite-1.

Problem is:

• Users in Site Collection's Visitor group (Assume "ABC Visitors") can access Subsite-1 but cant access Site 1.
• users getting this error:

  Sorry this site is not shared with you SharePoint 2013

Answer 1

Its working now! Users of "ABC Visitors" group can access the Site 1. Actually there is one custom visual web part which is added on home page of Site 1 & Code of this web part was not surrounded with SPSecurity.RunWithElevatedPrivileges.
So I got it working now by wrapping up the code with "SPSecurity.RunWithElevatedPrivileges"

Answer 2

I don't see any unexpected behavior here.
I'll assume you have actually three sites: ABC is the root site of the collection, Site1 is child of ABC, and Subsite1 a child of Site1.

You say "ABC Visitors has Read Permission over entire site coll" --> that actually means nothing, except maybe they have access to the root site (ABC). "Giving access to a site collection" is a misuse of language; actually you give permissions to the root site of the site collection.
So, in you case, they have read acces to the root, OK. But this access is not inherited by Site1, thus visitors can't acces Site1.
But they probably have been given explicit access to Subsite1, thus they can access it.
--> they can access ABC (root) and Subsite1, but not Site1.

EDIT
I'm editing my answer to insert the comment I had and that apparently led you to the solution:
Can user access another page than the home page of Site 1? It could be one component hosted on the home page, they don't have access to.