Without the form its kinda hard to tell exactly. But if i may go out on limb...

If you keep the cubes in constant water, it may be due to over watering/not getting enough oxygen. Those cubes can hold a lot of water and lockout oxygen.

pH of water? Are you adding nutes? Tap water, city, filtered? After my germination i only used pure tap water and added calmag for the first two weeks. The slowly bumped up the PPM until the plants looked fat and happy.

Also did you soak your cubes in pH'd water for a day or so. Those things tend to be pretty acidic.

Again, those are just the first things to pop in my head.