A lot of people struggle with this skill. There are many things that can contribute to you having problems. If it’s flooding often and your having problems clearing it, it may not fit you correctly. To get comfortable with nose breathing under water try putting your face in water without the mask and blow bubbles out your nose. After that try it with the mask. You can do this in a sink or tub. After that do the skills in a pool in an upright position. The person I got certified with struggled and we did this several times and they finally lost that nose plugging need while under water.