why a difficulty is still rising? No more devices were sold so far since the last batch of bitmain
No, there was a second batch of A3 that was delivered last month, a third batch of A3 that will be delivered in 2 weeks, Halong B52 miners that were delivered in the last 2 weeks, Innosilicon miners currently on sale (while it looks like they are the same as Halong, but branded as Innosilicon) and there is another secret manufacturer that we knew in January had plans to make Blake2b miners as they communicated with Core Devs (but don't want their name to be disclosed).
Maybe the obelisks are being tested before the shipping
Anyway, how Many of them has been sold?
About 6000 - 7000 units IIRC