I am knitting a Patons Shetland Chunky pattern, 15 sts= 4 ins with size 10 needles with a Reynolds Lopi yarn, 14 sts= 4 ins with size 10 needles. Will this substitution work? I’m having a hard time getting the proper gauge. I’m down to size 6 needles and getting 13 sts= 4 ins. I must be doing something wrong, improperly measuring the stitches in my swatch or something. The store I bought the yarn from said this substitution would work. Any suggestions will be appreciated.
That does sound like an awfully big needle difference. 
How big is the swatch you’re making?
here
are some articles about gauge that might be able to help you.
The swatch I knitted was 22 stitches. I took the swatch into the yarn shop where I bought the yarn and the sales clerk thought it was ok to keep reducing needle size. I just don’t think it’s right to reduce that many sizes. I’ll do another swatch with more stitches on the needle to see what results I get.
Could it be that the needles you are using are sized in mm?
Cause a 6mm needle is a 10 US size.
I’m using a size 10 US needle.