Please clarify if they are gates with jetways or parking stands with bus or walk-in.
If there's a jetway, a failure to connect it properly would cause this, and a very fast boarding with a message from GSX saying that.
If, instead, you are not on gate with a jetway and/or the boarding speed is normal and you are just missing the object, it's because you must have hit the maximum Simobjects limit, usually due to too many AI, too complex scenery or a combination of both, so you need to act on those settings to stay within the limits.