Make sure you are buying a display for the correct year; 2012-2014 A1398 LCDs are compatible (although 2012-early 2013 and late 2013-2014 assemblies use different camera connectors, the picture should work fine). 2015 A1398 LCDs will not be compatible at all.
Check around the LCD connector and the LCD cables of the assemblies to ensure you haven't chipped the cable or burnt any components. If you did not unplug the battery, this is very likely to happen.
Shine a bright light onto the screen to see whether you have a no picture issue or a no backlight issue.
Verify the assemblies you bought are definitely not faulty. There is currently a shortage of LCD panels for the A1398s, so it wouldn't surprise me if the not 100% new assemblies sold on eBay, etc are worse quality than usual.