Low EarningsPerClick - EPC [I don't know why they ever changed to the inexplicable Cost-Per-Click] generally is a symptom of low traffic, poor converting traffic, and even worse?

Low CTR.

While 18K might seems very acceptable to yourself in your niche, it might well be a drop in the ocean compared to many other sites in your niche. We don't know.

This is not a definitive answer, merely a long time observation. The truth is none of us really know the vagaries of AdSense. In any event? There is nothing any of us can do about it.

Have you made any changes to the site between June and July?

Some niches are seasonal,it is possible that after the beginning of summer there are fewer advertisers buying ad space in your niche, thus a lower CPC.

Is your site new?

as IanCP said:

The truth is none of us really know the vagaries of AdSense. In any event? There is nothing any of us can do about it.

traffic is the same as before i only changed the theme , all traffic is from usa and is from facebook,reddit,google and Rank in United States is 20,663

You should do some searching for the term "smart pricing" because it sounds like that is what might be happening. Things that affect your EPC include the quality of your traffic, and the perceived likelihood of conversion for advertisers. Some niches get plenty of traffic, but the people who click on the ads don't often complete a purchase or a signup from the advertisers. Does your site target people who are in some phase of a buying cycle? Those are often the ads that pay the best. If your site is informational or entertainment, those users might be less likely to convert.

It's all about keeping the advertisers happy.

traffic is the same as before i only changed the theme

can you change it back, and see what happens?

i did same again

Has your CTR changed at all in that time? Significant CPC drops usually indicate, as netmeg said, SmartPricing. That is a downward adjustment of CPC to account for poor converting traffic. This often follows an increase in CTR from accidental clicks.

ctr is about 0.5%

That is a pretty safe level, although best to compare with other units on that site (0.5% on a site that averages 0.1% might still indicate a problem).