Each and every time I change blades, I will inspect the blade holder to make sure there is no debris in it - which often times there is. I also will go through the motions for setting the proper blade depth, because I've learned that when I've changed blades, nothing good will result if I do not treat it like everything is brand new and needs to be individually tweaked.
If you're having a difficult time weeding, then there is a chance that you're not using enough force to not just cut through the vinyl, but through the adhesive as well. It could also mean that it's time to change the vinyl. Older vinyl that I have is notorious for needed a higher force, and sometimes a slower speed to get the results so that I can be easily weeded. Make sure that you're weeding whatever you cut sooner rather than later, because the adhesive will find a way to bond back to itself through the cuts over time.
Blade offset for Graphtec is 0
Since you have a Mac, I would suggest downloading and trying Graphtec Studio. It may not be your software of choice, but right now, you're trying to eliminate what it's doing, so let's see if it's a software issue vs. hardware.
I also highly recommend Clean Cut Blades.
The vinyl that I own are various colors and various ages, most of the vinyl can be cut using a single condition which is awesome, but there are some vinyl colors and ages that require fine tuning until it cuts right so it's easily weed-able. The fine tuning will include blade force and speed. Never underestimate the value of speed. The smaller or more intricate the text or design, the slower you go. I'm usually never in a rush when I'm cutting, and mutlitask while the machine is running, so often times I will be cutting at 30, maybe 40, but have slowed the machine all the way down to 15-20 for smaller stuff.
Good luck, and keep us posted on what's happening.