It's amazing when you look at the Middle Ages, and when it comes to monarch's power, there was actually a lot of to-ing and fro-ing between a king and his vassals on a lot of things.
A king had to rely heavily on his vassal lords to pay homage to him, due to how fragmented and decentralized the Middle Ages was, so lesser lords could get a bit of wriggle room in various ways.
King wants to go on campaign abroad? Okay, so he's calling up muster from your lands, but you don't want to get rid of all the men of fighting age. You live in an agrarian society after all; you need to keep some men back so they can work the fields. So you haggle the amount of men you will send on campaign in your name, either under your own personal banner or sending a knight banneret to fight in your name.
King needs food? All right, but obviously you've got your own manor to support and feed too, so you can't send him all the bushels and cattle and pigs he's requested. But you can send some.
This obviously doesn't mean a king was powerless: the response to the Hotspur Rebellion in 1402/03 against the new Henry IV shows that just because some lords might chose to fully withdraw their support from their king doesn't mean that all would.
Because a king's power is as much about the projection of physical and political power and strength, through armed forces loyal to the king and also through laws, but there's also that a lot of the king's power comes from a place of reputation and trust; about a vassal keeping faith with his king because said king, who he has met personally, has bestowed upon him his rank and title, deserves his support and his trust.