Thank you all for your help. I changed my needle - it didn't help. I believe I bought the Roc Lon brand at Joannes. I brought out my old Singer and it sews right through the muslin with no problems. What you have said about washing the fabric makes sense to me, so I will wash a row of 5 blocks that I have done and see if it is going to have the look I want. If not, I'll throw it all in the washer.