I've been working on the "On A Heater" achievement the last few days and it seems like the game has trouble keeping track of progress if you do too much in a single session. I've fixed it short-term by resetting my console and starting up again, but it would be ideal if this wasn't necessary.
Update: I finished it the other day, and I think I figured it out. I think the achievement timer only increases if you're in 4x multiplier but you're not actively in a sequence. If you're in a manual, or if you're in the air, or if you're grinding, it doesn't seem to count towards the achievement even though you have a 4x multiplier. You have to have all 4 wheels on the ground. This doesn't mean you have to keep letting the multiplier run out fully. First, get the multiplier, then just ride for a few seconds to let the timer start increasing. Then, before the multiplier ends, you can trick some more to add more time to your multiplier. Repeat until achievement unlocked.
