I have been using paytm from a very long time. I make lots of online transactions using this app.
But the main problem of this application is that sometimes due to loss of connection or slow internet network your transaction stucks in between and it has happened with me that the payment is made from my side but it is not recieved on the other side. And in that case the money that you have paid gets credited in your account again about 15-20 days after the incident.
So I will suggest you to check your internet connection properly before using this android application.
Thanks.