In my case the install callback seems to work intermittently. Over a period of 30k installs it will fail about 200 times that I know of.
I have tried my best to rule out the problem being on my end; I have four different servers, all of which run smoothly, all of which use different code to handle the callback.
I am estimating that the callback never gets called for about 5% of installs. I tested this myself by uninstalling the app and reinstalling. On the 5th or 6th try I noticed that the install callback was never contacted.
I'm sure you're familiar with apps that say "add this app to continue." For these apps, if the install callback is never executed, the app will never know you have installed, and prompt you to install again, which is impossible since you already have the app installed. In this case, a user is stuck and will probably assume the app is broken and uninstall. But occassionally they will send a message, and I go ahead and add their account myself.
This is kind of depressing when I think of the number of users who didn't send me a message and moved on.
Anyway I'm trying to rule out everything on my end, so if you have an idea please let me know, I'd be very grateful.
Thanks,
x