So no, Henry IV didn't like the Welsh. More than usual for an English monarch, he really detested the Welsh. Especially here, at his first Parliament, where those who'd helped him secure his throne were all looking expectantly towards their rewards, and the nobility were anticipating something to ensure their ongoing loyalty.
The second strike against Owain Glyndŵr was that - as he argued his case with all the eloquence of a lawyer and life-long courtier - the counselor standing alongside Henry IV, advising him on his judgment, was Lord Grey of Ruthin.
Henry IV sneered out his verdict, "What care we for these barefoot Welsh doggis!"
Even then, Glyndŵr did not rise. The royal blood of Wales met in his veins - two ancient ruling families enjoined by the marriage of his parents rendering him the true Prince of Wales by birthright - but he grasped reality only too well. He knew the might that could crush him at the barest excuse. Thus he returned home and let it go.
He still had property, land and tenants aplenty, and for that reason, he was legally obliged to raise a militia force when King Henry gave the word. Weeks later, when a Scottish army invaded the north of England, such a decree went out. All over England and Wales, soldiers were raised for the king's cause, but not on Glyndŵr's estates.
He hadn't been given the message. Lord Grey made sure it was never passed on to him, in the full knowledge that his neighbor's no show would be viewed as treason, and his land and property would be forfeit. Grey himself had designs upon that.
When Owain Glyndŵr discovered what had been done to him, he knew that his life was already over. On a very personal level, he'd just lost all. As soon as Henry returned from the north, the order would be given and that would be the end of everything.
In short, Lord Grey left the true Prince of Wales with nothing left to lose. That's why Owain Glyndŵr rose up, not at first for Welsh independence, though that's what his Rising became.
On September 16th 1400, he was crowned Owain IV, Prince of Wales, in a quiet ceremony at Glyndyfrdwy. Then rode into Corwen to publicly announce his fact, and to spread the word that all Welshmen who would rise with him should muster there.
Two days later, on September 18th, nearly three hundred people swarmed into Ruthin, looted its rich stores, then burned the town to the ground. The Glyndŵr Rising had begun.