One Problem can also be, that you are behind a proxy.
So there are two possibilities:
Add your proxy config to android Studio, ore you can also add a gradle.properties file in your project root.
there you have to enter the following credentials:
systemProp.http.proxyPassword=
systemProp.http.proxyHost=
systemProp.http.proxyUser=
systemProp.http.proxyPort=
systemProp.https.proxyPassword=
systemProp.https.proxyHost=
systemProp.https.proxyUser=
systemProp.https.proxyPort=
So the https Properties are pretty necessary. I figured out that often the repositories are available over both protocols. but sometimes only over http or https.