I was having the same problem: On first boot of the sim I would usually get the FL340, but after you explain Mach number I would never get the ATC prompt or the objective to slow down and the A320 would just keep cruising. I suspect it took too long to get to 340 and I overshot the trigger points. If I restarted the mission or went to main menu and then back into missions, I wouldn’t even get the initial welcome or complete the FL340, despite having reached the altitude.
On a hunch, I unplugged my Bravo throttle quadrant (that I wasn’t touching at start, but did use to turn the IAS knob). Mission completed just fine. So, could be related to something odd that the Bravo throttle is setting in the AP. Maybe try unplugging any AP or throttle controlling peripherals and see if that helps.