No, not all bags are like that. It depends on the bag, however, some of the Everlast Bags, and particularly the Nevatear Model Bags, have had a reputation for doing that. I believe it is related to the compositon of the fill (a mixture of sand and shredded rags). It is not long before the sand filters down to the base of the bag and compacts like concrete. Some people take the bag down and allow it to stay upside down for awhile before rehanging it, others beat on it with something, and other people empty and refill it with different composition fill or get rid of it all together.
If it continues to have a lower bag consistency like concrete, you can empty it out, try to filter some of the sand out of the fill, line the inside wall with a layer of closed cell foam (carpet underpad) and repack the bag with the rags that came out, and add some shredded cloth strips to fill the excess void. Some people also add a smaller diameter PVC material type tube to the inside center of the bag to add structure, however, that is generally on Muay Thai length bags.
Everlast seems to be marketing bags with better consistency fill lately, but you may not have one of them. If you choose to get another bag, the Fairtex HB6, and KO Fightgear 100 and 130 lb. bags, seem to be the preferred choice of upper end MT bags, and the best dollar value, lower cost bag, is the Title Classic Banana Bag at 100 lbs., and under $100.00. I hope this helps.
:icon_chee