Why does Restylane Lyft disappear two weeks after injections?

So many things to consider:
As we age our connective tissue changes, and can become heavier and may require more product to lift that tissue. 
What I have found in my practice is that there are some people that will metabolize filler quicker than others and may need to change the filler choice (i.e. radiesse, voluma these have higher G-prime and Vycross technology which takes longer to break down)  
I also found that when the Company states  the longevity of the product "Can last between 6-12 months" The actually amount of filler originally placed does not last that long (i.e. slow degradation of product)  
Other considerations on choosing the right product which may play a factor for cheeks, such as  the capability of the product to adhere to the bone (i.e. stay in place and lift). 
Cheeks typically need more product than most areas due to:  gravity, the connective tissue which we are trying to lift, and the limitation on how much fillers can actually adhere to the bone resulting in its lifting capacity. 
I hope this helps..  Good luck

Hi there! How very odd that you're having this reaction! I wish I had an easy answer for you, but we often don't know why something like this occurs. Sometimes it can be due to a "stressor", like being sick or mentally/emotionally stressed and the body seems to have a reaction and quickly metabolizes filler. Sometimes we just don't know! 
I have had success switching brands (from Lyft to Voluma or from Voluma to Lyft) when this has occurred. You may also want to try a long-lasting filler like Radiesse, a collagen-builder like Scluptra, or PDO threads to create the lift and contour you need. 
Hope this helps.
Best,
Dr. Kat