Verify_credentials Api call from iOS using Fabric Twitterkit not returning email always



I am developing an iOS app which has “login with twitter” and tweet from app, using Fabric twitter kit to login. My App is Whitelisted for getting email ID. After successful login, I am calling to verify credential API as below:

let client = TWTRAPIClient.withCurrentUser()            
 let request = client.urlRequest(withMethod: "GET", url: "", parameters: ["include_email": "true", "skip_status": "true"], error: nil)

I am getting email ID for the user’s first login and the user account get configured in iPhone System accounts.
We have logout feature in the app, for logout I have used below API call:

    let url = NSURL(string: "")
    let cookies = NSHTTPCookieStorage.sharedHTTPCookieStorage().cookiesForURL(url!)
    if cookies?.count > 0 {
        for cooki in cookies! {

If I login again in the app and call verify credentials I am not getting email ID for the User in response.
Even I tried with:

    var client = TWTRAPIClient()
    client = TWTRAPIClient.withCurrentUser()
    client.requestEmail { (respStr, clientError)

Its returning Email Id not found for the User, even if Email Id is verified and for first Login it was return from APIs

Issue is I am not getting emailID consistently for the same user.

Please let me know, what is missed in the API calls or is there any other way to implement it.
Help is appreciated.