OK, here's the 1st day of shooting with this instant back and I got some tips for you. If you haven't picked up your LCA lately, make sure you check the battery level coz without good battery you are bound to screw up any photos taken with it. Press the shutter half way and see if the red LED light up responsively, if the LED flicks, change battery or clean up the metal connections.
Secondly, get familiarize with your LCA's distance switch once again. 0.5m is approximately the distance from my finger tip to my elbow, so I am confident that distance is not a factor contributing to any of the blur imagery. Finally, in lower light condition, make sure you don't move too fast after pressing the shutter, you should hear the second click sound then you are free to move.
I have to say the LCA Instant Back+ is a great product, once you are fluent with the above tips once again, you are free to explore other techniques such as double exposures without worrying too much wasting precious films. This combination creates very strong vignettes, and in most cases they look great. Applying a wide angle lens on your LCA will create less vignettes but the result is more distorted image.
BTW, my experience told me that using a flash at ISO 200 will probably create over-exposed image. I suggest you to shield the flash with color filter or simply put a thin tissue over the unit. I'll be interested to hear your stories and experiences.

