Rubble is correct.
Let me explain how buses work:
Every single bus stop has a bus rotating on a predetermined schedule, regardless of whether there are any passengers or not.
Although the buses can cruise empty, they cannot exceed their maximum passenger capacity.
The issue might be that there aren’t enough buses cruising to supply enough PAX quickly enough. The only way to add more buses is to add more bus stops.
Be aware, however, that bus traffic might also become an issue, especially if you do not space your bus stops correctly. Buses ALWAYS come at predetermined schedule, regardless of whether the previous bus vacated the stop or not, which is caused by inefficient airport layout, as a bus will NOT leave until all assigned PAX are inside and will wait for the ones on the other side of the terminal.
Alternatively, stick a metro entrance - it should alleviate the situation.
Car stops are inefficient in terms of capacity, but they add to realism.