Very hard to join a PPI network ?

Hi, I easily joined RevenYou, Amonetize and perinstallcash. But I am looking for other competitors. I signed up for all the following networks and none of them responded!
-InstallMonetizer
-CodeFuel
-BetterInstaller
-OneInstaller

and few more but no response at all ! Even thought I have almost 2k installs daily.

Is there some tips for how to join them ?

I also tried to apply to better..., and after a week or so I sent emails and within a 24 hours I got their reply - unfortunately negative. As for the first one you mentioned , after a week with no reply, i sent emails and I put my s kype in emails, so after a few hours they contacted me on s kype and i got in. Sorry but they will not allow me to post those e mails, you can pm me for them. Good luck!

What`s the point to sign up to all the PPI networks.Register to 2-3 and stick with them.

I got approve by InstallMonetizer. Didn't try for any other network yet. but, when I try to contact with skype, they ask me to show YT views ( I mention YT as my monetize method ) then soon after they instantly approve me into InstallMonetizer.

I think it is good to work with multiple networks, to at least see what works best for your traffic. A question you have to ask yourself of course, is if it makes sense to work with a network that initially already doesn't have the time for you. I mean, if they don't even care to respond on your application, do you think the account manager(s) are different when it comes down to answering questions?

However, as you asked for tips, I suggest to do a simple Google search, taken as example Amonetize (I am not vouching / promoting them, just using them as example. Go to Google and search:
amonetize inurl:linkedin account manager

It will give you the names of people working in the company. Pick one and add him or her to Skype. This works pretty well for me. Each serious company has Linkedin pressence, in my opinion

simple tip... during application process you have to tell them exactly how are you going to make money with them and be ready to provide evidence like URL of SW landing page or traffic sources and so on. Than there should be no issue with accepting.