I always use Walgreens BD Brand 30 gauge 1/2 inch or 5/16 inch depending on where I plan to fish or what is next in the rotation. I am very particular about rotating and letting viens heal properly. I know exactly what you mean by not wanting to lose some of the product, sometimes no matter how I flick my rig there is always a tad bit hanging around the reservior next the needle. If you are pulling back on the rig and is it retracting to its original position without force you may be in an area of your body that facilitates this action. I know exactly what you mean, I have a lot of scar tissue from a previous miss + abcess + methacillin/vanomycin resistant staph aureus infection and if I inject anywhere within a 2-3 inch range of the crook in my left elbow (bicep, upper forearm) the rig will ALWAYS retract to its original position on its own. What will happen with me (never been a user of tourniquet and I have been IV'n for 4+ yrs, just use a space heater or warm water to wake them up, roll up my sleeve and knock on em a little to make sure they're home and ready to go) is I will position the needle, bevel down and steadily ensure that the position of my tip is constant and steady as possible, when I register I will get the usual fluff of crimson red, sometimes not (just a failure to register while the rig returns to its original position)... Either way it will always return to its original position when I inject in this area of my left arm, I will register, watch the blood and about 5 units of solution sort of "auto-inject" and will manually, albeit stable as possible, finish the plunging myself. Are you noticing this while trying to register in multiple places on different extremities? You are essentially creating a vacuum within the tissue you've placed the needle and if you are relatively skinny or have a higher precentage of muscle I could see this happening. Are you having a really hard time registering at ALL, ANYWHERE? If so my first guess would be dull tips but then again you've got brand new ones. Are the locations you're trying protruding for easy lineup/entry? I know I rambled and was not of much help in solving your issue but I can definately relate, having experienced the same thing myself. I wish you the best as I am sure you're fucking frustrated to all ends right now. If there is a will, there is a way.
P.S. with my g/f I have always used a rather thick, larger rubber band as a tourniquet for her. Cut so you have a long straightened band, place above the site you wish to use etc etc.
Good luck dude!