Android :: Misses Periodical Execution Of Thread Using ScheduledExecutorService

I have an android app that repeatedly collects fingerprints from the wifi-networks that are around (for scientific reasons, not to invade anybodies privacy). Anyways, imagine I have a function that does this work and it's called scanWifi(). I initally wanted to start it like this:

ExecutorService mExecutor = Executors.newSingleThreadScheduledExecutor();
mExecutor.scheduleAtFixedRate(new Runnable() {
@Override
public void run() {
scanWifi();
}
}, 0, interval, TimeUnit.MILLISECONDS);


Sadly, this only works reliably when the phone is plugged in. If it's plugged out and lays there for a while, it doesn't run my scanWifi() function every minute. Sometimes there are gaps of several minutes between single calls to scanWifi(). I also tried doing the same thing using a Timer/TimerTask with similarly poor results. The only thing that seems to work more or less reliable until now is to post it to a handler and call it repeatedly, like this:

Handler h = new Handler();
Runnable r = new Runnable() {
@Override
public void run() {
if (!mIsStopped) {
scanWifi();
h.postDelayed(this, mInterval);
}
}
};
h.post(r);

Why is that the case? Is the CPU sleeping and thus misses the scheduled execution? I hold a partial wakelock in my app.