Some toilets have a shorter horn on them, which is the ceramic ring on the bottom of the toilet that initially contacts the wax ring forming the seal. Some toilets have longer ones and some have shorter ones. Combine a short horn with a standard wax ring and a toilet flange that is slightly below finish floor, then you have a potential leak. One piece toilets are heavy so it was quite possible that you didn't even feel the horn contact the wax ring and give that "squish" that you should feel as the toilet is being set. Or, that heavy toilet has a longer horn and you squished all of the wax out. Or you may have just missed a complete contact with the ring when you set it. Pull the toilet and look at the ring, if you made complete and full contact, then the ring should be evenly squished with an indentation of the toilet horn. If not then you may need a thick ring or double up on the standard rings. I've never had an issue with wax rings that have horns in them. When I need to make up a big gap I'll pancake a horned ring on top of a standard ring. Unless your flange is sitting very low, I wouldn't mess with it. Flanges, ideally should be set flush if not ever so slightly above finished floor but it is better to have a flange a bit on the low side then a flange that is set too high. For a DIY guy, changing the height of the flange should be the last option, unless it's visibly damaged or your flange is set too high, in which case that heavy toilet is squishing out all of the wax. So, basically, the flange is too low or too high, cracked or you missed full contact with the ring.