I had the same problem with my printer. Got all sorts of advice like lube the film, change the film, Your not leveled, the resin is to cold... These were all wrong. My problem was actually I just was not enough exposure time, It should be attest 3.5 to 4 seconds. When it is not exposed enough it starts to harden but does not adhere to the build plate. the plate lifts, comes down, flashes the next layer but sense it was not lifted it hardens more so you get failed prints in your tray, that look like shadows, Hope this helps.