try a poly invisible thread- it will make a huge difference- the nylon- stretches, snaps, melts, becomes brittle and breaks with age- is really not a good choice for quilting= becomes very (poky) when it breaks
polyester stays soft- does not melt-stretch or become brittle.
visit Superior Threads web site they have lots of free reference materials on threads of all kinds- what needles to use and what each thread is good for...they have free charts you can print off to keep handy for threads and needles.