Depends on the niche, ad placement, quality of traffic and more. Could be any number of things. Maybe you are in a low paying category. Or maybe you are not catching visitors when they are ready to buy. Or maybe Google does not trust your traffic and is reducing your clicks and/or reducing your epc.

My first guess would be problems with what the site is about.

There are some topics where advertisers won't find it productive to advertise, and they'll only make dirt cheap bids if they bid at all. Some might even be actively excluding the topic or even the individual site from their placements.

My second guess would be that there's not enough advertisers bidding to reach audiences in Greece.

How are the ads positioned on the page?

Thanks for your answers both ! Can I post a link from the site here i order to see the placements of the ads ?

No links or other specifics, please. You can get a site reviewed if you are a contributor, but otherwise, we don't do that here.

I'd look at two things - test some alternative ad placements, and take a good long look at who and where your traffic is coming from.

Thanks anyway for your help.

Sometimes the metrics will answer your questions.

Do your visitors click the ads? Average CTR is 0.8-2% so 600,000 page views should be yielding between 5,000 and 12,000 clicks per month.

If you're getting that many clicks but only making $120 a month, chances are your CPC is very low. In other words your ads aren't paying.

Simple solutions. For low CTR, review ad units and placement. For low CPC, review your niche and/or attract more traffic from US, UK, Canada and Australia.