Check the stem valve of the tube and see if it is tight or defective. I had a tube valve body come right out of a tube once. People take them for granted but they can be defective and have slow leaks or get dirt stuck in them and leak down. And yeah, that slime stuff they sell to put into tubes is gimmicky garbage. The right way is to carry a spare tube or patch kit and a pump with you. The goo in the tube just complicates things, throw that out and go get some nice used tubes out of the dumpster behind your local bike shop.