For 27's I go to walmart. Their prices are good and even they can't mess up on tubes and they even carry the hard to find 26x1 3/8, For 700's I go to my LBS.
Make sure that you aren't pinching the tube between the tire and the rim, a very common mistake. Wneh placing the tire back on the rim, you should be able to do most, if not all the work by hand with little help of your tire tools.
Run your fingers along the inside of your tires to feel for any sharp object that may be lodged in the tire, even an unseen thorn can cause problems.
Ernest