Well it depends where your putting the coal also. I noticed it when I first started doing hookah that the top part of my Shisha would burn but the bottom was still good (I was using an egy. bowl) So a friend of mine told me to work it around the corners and not place it in the middle, make tiny holes so ash doent get in, and dont over pack. You just gotta work with it, try different ways and see what works best for ur hookah.
I also noticed that if you have a small bowl and use coals like Exotica or any big coals, it tends to be tooo hot so it completely burns it quick...so on small egy bowls I use Starbuzz coals or 3kings on even the quick lights that most hookah lounges use...Its a main reason why I switched to a funnel bowl and I love it. Last longer too.
Hope I could help