My greenhouse wasn't sealing because of *one* cat tail growing in a block of water next to the building.
I actually went so far as to build a "mini" 3x3 greenhouse to make sure it wasn't the doors or glass slab placements. When that worked, I ran the perimeter and found the one cat tail on the back of the building.
Remember, glass slabs go in the lower portion of the blocks over your green house. You place them this way by looking at the very bottom of side edge of the block before clicking.