Concurrent Service Calling in WCF Rest API.

hi i am working on WCF Rest API, and developing Service for the mobile application. Now the problem is as below.

 

There are three device or mobile from which we send some request.

 

Device A, Device B and Device C.

 

There is lot of service hosted. but for example lets take service name ..X(), Y(),Z(),Ay() etc. 

 

now at the same time A device call X(), B device Call Y() and C device call Z(), then some concurrent access problem is there, service take time to respond.

 

But when we call service from single device then its working ok.

 

So what i can implement please suggest me.  

 

Thanks.