Sewing class

I have been practicing putting a zipped pocket in the material that is the same as the  trousers I am making. I am happy with the pocket and I have tried to put patch pockets on the back like you get on jeans. The patch pockets have been a challenge as the thread keeps looping under the stitching even though I have changed the tension. I think this is because I have used iron on facing on the back. The teacher said I could try a specialist needle and see if that solves the problem. I bought a jeans needle. I might need a specialist stretch fabric needle. I will give it a try later. Any advice would be welcome. The fabric is scuba and I have not sewn it before.