There was a recent posting on machines being fussy with thread. Unfortunately, it seems that it's not the brand of machine, it's the individual unit. So one Juki might like on thread while another Juki may prefer another. I would suggest starting with the Connecting Threads thread. Get the smallest spools available, in a neutral color, one each of the poly and the cotton and try them. If they don't work, you can use them to piece. If you live near a JoAnn's you can use your coupons to try different brands. Good luck. Hopefully, you will find your Juki is not fussy.