I had this behavior too that it got stuck at 6. But my plane "jumped" on the ground, so I think this was the cause. Now at another airport, where I havnt got a low-riding 707, it counts below 6 very nicely!

Dont know what my FSX did, maybe reloaded the scenery and afterward my plane startet to jump. Or it might be a problem with the add-on scenery itself due to a higher ground-level than the original FSX-standard-airport...