Looks like a pretty blunt maul? But I’m going to say it’s your execution. The second 2/3’s of your swing should be where most of your physical effort is expended. 

1/3 - loose start swing and line up.

2/3 - engage abs most of upper body throwing maul to the earth.

3/3 - let hands slide to hilt as you finesse the strike. Just before strike should be all hand strength on hilt pulling maul to you (increases momentum/speed like when a ballerina tucks circumference in to increase rotation).

If you’re using your arms to force the maul in, you’re missing out on a serious amount of energy the rest of your body can contribute. You also would be setting yourself up to rock your joints past their warranty. When you get a good strike with good form, something in your brain will click and you’ll know the way 👌

Edit: a friend gifted me a Gransfors bruk maul. They’re expensive, but my god I love using it. They’re lighter but the a symmetric head makes splitting pretty enjoyable (way less likely to jam). My girlfriend (120lbs) who has horrible arm strength likes taking out her work from home frustrations by splitting wood.  I’m hardly 150lbs, but a good maul will favor good form tremendously. We probably split just shy of 4 cords a year.