1) The DWDM SFPs have a high TX power as they are long range SFPs. Hence attenuators have to be used while connecting them. If attenuators are not available then the cable into one of the SFPs need to be loosely inserted. Only then will the link come up.
2) The GLC-ZX-SMD SFP from the Finisar vendor also has a high TX power and needs an attenuator (or else the cable should be loosely inserted into one of the SFPs) to bring the link up. However GLC-ZX-SMD SFPs from the Sumitomo vendor do not need attenuators and come up fine when cables are properly inserted.
3) GLC-BX-D SFP need to be connected to a GLC-BX-U SFP to bring the link up. The same applies when interconnecting GLC-BX40-D-I and GLC-BX40-U-I as well as for GLC-BX80-D-I and GLC-BX80-U-I. Basically the D sfp should be connected to the U sfp.