If dyeing with fabric dye does not work at all, the most likely cause is that the material of the fabric is polyester or highly water-repellent. In incidental cases the cause can be found in the operation of your washing machine.
Polyester is actually the only fabric that does not give any result after dyeing. Polyester requires a very high temperature (approx. 190 degrees) and / or a special type of dye. You cannot dye polyester with normal fabric dye. So always look at the label of your fabric to rule out that the fabric is made of polyester.
Many fabrics are also made from composite materials. A fabric that consists of 30% polyester and 70% polyamide will color for 70% percent. The polyamide (Nylon) does color, the polyester does not. This usually means a lighter end result or occasionally some parts that are not colored.
Another possibility is that the fabric is very strong and water-repellent. You can easily test this by dripping a little water on the fabric. If the water does not penetrate the fabric, you are dealing with a water-repellent material. Water-repellent materials also do not absorb the paint.
A less common cause is possible if you dye in a washing machine and you poured the fabric dye directly into the drum. Some washing machine types start a washing program by draining residual water from previous washing cycles. You can also hear this pumping out as soon as you switch on the washing machine. If this is the case for you, this could be the reason that most of the fabric dye has disappeared before dyeing started.