Divide your write up in different capsules. Hubpages will automatically add ads in between capsules. You don't have to do it. You can find capsules in the edit mode at the top right side. Select text capsule for inserting text. It is advised to divide your text in more than one capsules.
If you put entire article in once capsule, then ads will not appear in the middle.
Please ask this type of questions in the forum. Or search old forum posts for answer to such questions.