I plug my meth once a week. One dose only, no redosing so I couldn't tell you if the previous dosages could be affecting but I can tell you how I effectively plug. Fully dissolve in water of course. I too have a 5mL syringe with almost a 1 inch long tip. I dissolve my gear in an old vodka cap. Nice and small and easy to manuever.
Anyways lube it up and everything as you said. You only want to go in just enough to where it won't come back out. 1-2 inches. Any further and you run the risk of shooting it into the colon, and not the veins in the rectum. You want to push it slowly to where it doesn't shoot into your small intestine, but fast enough it doesnt just dribble out and run down the side, possibly coming back out.
Try practicing to get it down. After the plunger is pushed all the way down leave it there for like 10 seconds or so just to be sure none dribbles out. Slowly pull it out. I usually put a mL back in the cap to make sure I got everything. I usually sit up at an angle that way it leaks back down if I got any too far. Anatomy shows that the left side has a deeper crevice so lay on your left side, unless you are left handed.
If you follow these suggestions and it still doesn't work, make sure your bowels are empty. They will soak up most of the water. I usually take a dump, take a shower, then wait a good 10-20 mins to make sure nothing more is there.