Unable to find a specification for `PrivatePodBB` depended upon by `PrivatePodAA`

I have created 2 private pods first one is PrivatePodAA and second one is PrivatePodBB , I can successfully use PrivatePodBB in PrivatePodAA podfile but while trying to add it as dependency in PrivatePodAA podspec file and trying to install PrivatePodAA as a pod in new project it's showing :

Unable to find a specification for `PrivatePodBB` depended upon by `PrivatePodAA`

I have created my own private repo and added PrivatePodBB podspec file in it.

Below my podfile for PrivatePodAA which is working perfectly :

// my private repo url
source 'https://privateurl.prv/scm/ios/sdhub.git'
target 'PrivatePodAA' do
    pod 'PrivatePodBB', :git => 'https://privateurl.prv/scm/ios/PrivatePodBB.git'
    # pod 'PrivatePodBB' # this also working fine
end

And below podspec file for PrivatePodAA which which causing the problem :

Pod::Spec.new do |s|
  s.name             = 'PrivatePodAA'
  s.source           = {
     :git => 'https://privateurl.prv/scm/ios/PrivatePodAA.git', :tag => s.version.to_s
  }
  s.dependency 'PrivatePodBB'
end

So, please what is missed from the above code ?