Evening, W8lfter,
I had exactly the same problem with my own 642. However, I had already been using HKS speedloaders with my old Model 60, and also a very old Model 36. That Model 60 has been wearing Uncle Mike's-made boot grips for many, many years, and has never bound up, or hung up. So, I removed the S&W-marked/made boot grips from my 642 and replaced them with my old Uncle Mike's version of that same grip. VOILA! Problem solved. So, emboldened by my success, I found a pair of NIB Uncle Mikes J-frame boot grips, on 'the Bay, and bought them. Like my own older Uncle Mike's, the NOS ones fit my 642 perfectly, and the HKS speedloaders now work just fine with that little shooter, too, just like they do with my M60. For what it's worth, I think that, when those S&W versions of that grip were made, something changed just enough to cause the problems that you are having, and I had. Try an older pair of Uncle Mike's boot grips, and I'm betting that your 642 will work just fine with HKS speedloaders, just as did my own. Take care, and God Bless!
Every Good Wish,
Doc