Sometimes there is no sound on the count in, or its very low. I doesn´t matter how I adjust in the count-in/metronome menue. What´s wrong?

Welcome to the forums.
What version of Band In A Box are you using?

When you say 'sometimes', are you able to identify specific examples when this occurs or is this a completely random event?

Is it the entire count-in or just the first part of the count-in?
I ask this last question because on my main system, the first beat of the count-in is always missing or very soft in volume. This is a known problem (to PG Music), but has never been fixed.

The sound of the count-in can come from midi drums or Realdrums. For midi drums the sound is generated by whatever you have set up as the default midi sound module. For RealDrums the count-in sound might be audio associated with the Realdrums or midi when Realdrums audio is not available.

The volume level for the Realdrums count-ins varies greatly. Some are barely audible while others are very clear.

There are volume level settings in preference but preference settings affect everything, not just the count-in.

I don't have a fix but I thought you might like to know a little more about the count-in.

As mentioned the count-in can be Audio or MIDI.
One of the advantages of RealBand is the ability to visually see the audio (if it is coming from Realdrums) and simply increase it with various tools.
If it is indeed MIDI, RB again makes it easier to edit the few count-in notes for volume.

If it is from the Metronome though, both programs have a habit of using the default synth, which throws the third variable into the mix.

Is the count-in from the Metronome, or the generated song?

I've been searching the forums, and I see no fix for this, just various work-arounds. If using RealDrums, the midi count-in is virtually inaudible, no matter what volume setting is chosen in Preferences for the count-in.
Interestingly, in the piano-roll view, one can edit the volume of the count-in, and when clicked with the cursor, those piano roll count-in notes are louder. But on actual playback, no change on count-in volume occurs.

Have you tried:
1-options
2-preferences
3-count-in/met
4-Drum count in options

Note that I have mine set for B substyle fill in. You have other choices from the drop down menu.

If it is intermittent, it's possible that it might be style-related. Next time it happens, make a note of what style is loaded and post the name of that style here in the thread. This will allow others to test it.

This issue is also being discussed in the Windows Forum.

I gave details how to quickly and easily edit the count-in regardless if it's generated by MIDI or a RealDrum in BIAB without having to export/import the song's drums track to a DAW or Audio Editor Program.

A link to that post and you also need to read my following post beneath it.
https://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=661178#Post661178