Tires will not last forever and they are also not a perfect creation, just like everything else. As such, specifically with tubeless mountain bike tires, they may be a little maintenance required on your part. Unlike actually installing and sealing them, this is nothing you should fret over as it is fairly basic stuff. First off, always make sure to check the tire pressure of the tires before you take off. But, remember that lower air pressure can actually aid in control. Next, if you are noticing that the tire will simply not hold air, you may need to remove it, clean the wheel rim and reinstall the sealant. If you took your bike to a shop, inform them of what a terrible job they did!