"The site collection could not be restored. If this problem persists, please make sure the content databases are available and have sufficient free space."

Hello Experts ,

In my scenario I am having one Web Application under that around 6 site collections I have created and all sharing same content database. 

The same type of sites I configured for my QA and Prod environment. Now I have done so many changes in one of the site collection are per our requirement. 
To move that site collection to another environment Prod I am using Restore-SPSite Command from SharePoint Power shell and It is giving me below error .

"The site collection could not be restored. If this problem persists, please make sure the content databases are available and have sufficient free space."

command which I used :

Restore-SPSite "{servername:port}/sites/SubLevel/" -Path {backuppath}\{backupfilename}.bak –Force -DatabaseServer {DatabaseServerName} -DatabaseName <ExistingContentDB>

I checked my disk space and it is having more than required disk space .. So I dont think so error message is correct .. 

Apart from that I tried a way in which I Created new Content DB under and restoring using below command then It working fine ..

Restore-SPSite "{servername:port}/sites/SubLevel/" -Path {backuppath}\{backupfilename}.bak –Force -DatabaseServer {DatabaseServerName} -DatabaseName <Name of New Content DB>

But that solution is not fulfill with my requirement . I don't want to create new content database everytime ...Any expert comment on same issue  ?

Articles which I already refered as mentioned below : 

https://support.microsoft.com/en-us/kb/926061

https://goo.gl/IURGHN

https://goo.gl/Ac83A0

http://goo.gl/nsg9oc

https://goo.gl/WPhiZI

http://goo.gl/a16bVO