Smoothing issues on a symmetrical model

[Anonymous]

Hello,

I am fairly new to Maya and today I encountered a problem regarding to smoothing. 

Even though I am working symmetrically, smoothing the object gives a different result on both sides, somehow it's creating weird artificial holes at the back side of the model (See screenshots).

Is there maybe a logical explanation to this? 

Thanks you advance for the help 🙂

[Its_jay]

You most use (bevel) option near the smooth area
Than smooth it