Would you be surprised to know that you may be doing absolutely nothing wrong? Even though you tried to sanitize your corks (assuming you are using natural corks and not artificial corks) by soaking in a sulfite solution before bottling, it’s almost impossible to completely eradicate all of the organisms that could cause TCA (trichloroanisole) in a cork. The main reason is because the mold spores might be inside the pores of the cork rather than on the surface where your sanitizing solution reached. There are also many other ways that TCA can crop up later in bottled wines. It should also be noted that there should be no need to sanitize corks from new, unopened bags since these are shipped with SO2.
TCA, or the “corked” off-aroma, is caused by ambient molds interacting with a chlorine molecule of some kind, usually from domestic water supply. When these mold spores metabolize anything containing chlorine they can produce the noxious and easy-to-smell TCA molecules. These mold spores are frequently found in corks but they also can live in drains, on stacked cardboard in the winery, on pallets, or even in the air you’re breathing. Controlling all of the mold spores that might possibly create TCA is quite a daunting task.
I would be curious to know how many bottles were/are affected. If it’s only one bottle, then it’s possible it was the closure, assuming you’re using a natural cork product. If it’s every bottle, then the chlorine-fungus combo that caused the TCA possibly happened because of one of the other scenarios.
This is why most winemakers focus on controlling the level of chlorine in the environment and is why it’s critical to never use chlorine bleach, or any other chlorine-containing cleaning products, in a winery. Make sure that even the towels or rags you use for cleanup are not laundered with chlorine-containing soap. Additionally, always try to use chlorine-free water for winemaking. Commercial wineries have huge filtration set ups but home winemakers can get away with something as small as a chlorine-removing showerhead on the end of the hose. Most hardware and home repair stores stock these. You can also find affordable and slightly larger-scale systems, including manifolds of wall-mounted filter cartridges, with industrial supply companies like Grainger.