Have you tried starch? Are you using 100% cotton? Sometimes thicker fabric can create a thicker seam. How hot is your iron?
Not sure what you mean by "wiggling" but in case you "iron" by moving the iron back and forth, that's not the best way to do it. In quilting we are supposed to "press" so we don't distort the pieces. Wiggeling sounds like it could distort and then you're lumpy and crooked.
I usually set my seam, then press one piece over. If it doesn't lay flat, I give it a burst of steam.