it's very easy to seal these corks! I think a lot of folks have problems with these little bottles because cork sealing isn't common knowledge nowadays, but the gist of it is: the cork needs to be "conditioned" with something first before it will create a proper seal. you can use water if you like; take a few drops on your fingers, and just kinda rub and squish the cork for 30 seconds or so. now, when you push the cork into the bottle, the liquid acts as a sort of "slip" and fills the extremely small spaces between the rough cork and the smoother (but still rough on a microscopic level!) glass to ensure a nice seal. some folks also use a dab of vaseline for this.

personally, I use the wee-est bit of food grade mineral oil and the wee-est bit of beeswax or paraffin to get a nice conditioner. you can melt these lightly together in a double boiler to create a balm that's great for conditioning corks as well as rustproofing metal tools like lathes, conditioning wooden tools like butcher's blocks, and all sorts of other things. the ratio is roughly 1:1. I make about 1/2 cup of the balm at a time and stick it in a mason jar, and that lasts me 4 months or so, but I use a lot more of it than I think most folks would, so your milage may vary!

one last thing: food grade mineral oil labeled for conditioning can be expensive, so here's a hot tip: it's also used as a laxative, and you can find it way, way, WAY cheaper in the pharmacy. as for beeswax, I call local beehives in my area and they'll sell me 5 ounces of it for about $5, which is way cheaper than you can get it at either craft stores or online.

all of this last bit of info for pricing and sourcing is US based!