That can only be caused by four things.
1. Printing on a fabric other than 100% cotton.
2. Not enough pretreatment. I suggest you thoroughly soak a shirt, dry it by heat press for about a minute and then try to print. I mean really soak it.
3. The head not laying down enough ink. This could be several things. Clogged nozzles, which you say are fine. Incorrect resolution. You should be printing the under base at 1440 x 1440. The ink levels are turned down in the RIP. They should be between 85-100% for the white ink.
4. Bad ink.
If you really soak a 100% cotton garment with pretreat, print at 1440x1440 with the white ink levels at 100% in the RIP, after printing a solid nozzle check and it does not puddle ink on the shirt, then I would have to say bad ink. However I really suspect it is one of the first three items, with emphasis on bad nozzle check or not enough pretreat.