Keyword Rankdrop

[PawanKumar]

Hey Guys,
I have been working on a website. My 6 keywords were ranking on 1st and 2nd pages. But, In last month i.e May the ranking started to drop and it one of my keywords felt to 70. I check the webmaster if there is any penalty on my site but, it was clear. I checked for algorithm updates "The core update" but I was aware of the core updates. I don't know this might be the problem.

I issue I got is Mobile Usability issue on my site.
Can this be the issue hampering my site?

Please help me.
How can it get the ranking back?

[raghuramastrologer]

 Google changes their search engine result pages, often rank trackers need to update their software. This can impact the rankings they report to their users. If you don't find anything on your rank tracker's website, check out their Twitter account too. They may have tweeted about it, or have received tweets.

[Yuga]

Most frequently, we see drops because of changes that were made to the website, but they can also be caused by an algorithm update, technical issues, improvements competitors made, SERP lay-out changes or a Google penalty.