Is your cartridge going all the way into the 1200XL? Make sure that you clean the 1200XL contacts, also. A pipe cleaner with contact cleaner does this well, although that recessed cart port can be tricky to get into to clean. If it doesn't work after a good cleaning, I'd re-flash and then if it doesn't work, set up the drives again using FDISK2.XEX. Or, you might have a flash card issue. I've used the combo you mention, and it works well for me. BTW, the 4.3e rom works well with the 3.5F version, including Hias' excellent HSIO patch..
I dug out a 1200XL to play around with, but I can't get it to boot from my cart. I get the opening menu (the 2008 OSS special with Pico dos), but selecting any of the cartridges gets me nowhere. If I select Pico, it sort of works, but files and drives are missing.
Any ideas? This machine does have the Newell 256K upgrade.