Fluidline dries up, thumbsuck for reason

I use Mac's Fluidline eyegel.I am on my 3rd tub.The 1st 2 tubs,dried up when it got to the last bit (enough for a lot of uses).I am from Jhb but bought my 3rd one at Gateway Mac store.I asked the sales consultant why it is drying up.She firsts said that this is news to her,but I then told her it's happened to me twice already.She then said....when it comes to the last bit inside the tub,the slightest air can cause it to dry up.So first she didn't have an answer & all of the sudden she did.With that being said,no air got into my tubs.I am so careful with it cause when I first bought it,I was explained to close it properly.So I do cause Im not going to waste money by being reckless.I got rid of my first tub,but when the 2nd one did the same,it gave me a red flag.Why is this happening?I am not going to be accused again for not closing it properly as I know that I do.There is also a girl from gym that uses the same thing & exactly the same thing happened with hers.Please explain & maybe train the staff that sells the product to know what is happening