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I have a bit of a problem. My largest dragons have a flight range of over 1000 miles, but I have another continent that is only reachable by ship significantly closer (just over 750 miles). I can't use some excuse about winds, as my ships are mostly sail powered.

Further hampering my progress is the fact that my dragons originated on this other continent, as did humans. I need some way my dragons flew off the continent, but can't get back, that also makes it possible for ships to go back and forth.

NO MAGIC MAY BE USED.This is because my only magic is the dragons themselves.

The wind isn't such a bad excuse.

Say there is a prevailing headwind, which for most of the path is faster than the average speed of the dragon. Dragons can fly one way but not the other.

Your ships, however, have a keel and triangular sails. These two inventions together permit a ship to sail upwind, tacking back and forth. This revolutionized the maritime world when we figured it out! As long as we can make sails strong enough to withstand whatever speed wind is sufficient top stop the dragons, we're set!

The only loophole I can think of is a dragon building a ship with a keel and sailing upwind. However, this becomes a different question: how does this fictional plot device you need tell the difference between a human crewed ship and a dragon crewed ship!

Dragons can't swim.

Your dragons can fly 1000 miles but they cannot see 1000 miles. Nothing can. On the ground you can see 10-15 miles. In a plane maybe you can see 150-200 miles. If a dragon sets out over water and it sees only water in front of it, it will turn around before it loses sight of the land behind it. A flying dragon wants to always have some land within view because it knows that if it gets tired and has to come down over water it will drown.

Your dragons originally came over because

1: Sea levels were lower and there were islands along the way.

2: It was an ice age and floating icebergs count as land.

3: The dragons of old were not as big of wusses as modern dragons, and were willing to fly over open water without lips quivering in fear. They drowned a lot too but that is part of not being a wuss.

Dragons, Like Birds, Navigate via Magnetic Markers

Part of the sea near the far continent has something that causes the magnetic pull of the earth to behave erratically. Or add in some sort of electro-magnetic disruption that messes with their heads in a way that doesn't affect humans (much). It is situated so that it is impossible to go around it without adding at least 500 miles to the voyage.

This poses no problem for ships as they can use the stars to navigate well enough until their compasses work again.

But the dragons can't function there. They avoid it instinctively. Any dragon who gets close feels so uncomfortable (or even in pain) that they turn around and get the hell out.

The home continent has been struck by a species of fungus or bacteria that is highly lethal to dragons.

It can't survive in the ecology of the other continent, but any dragons that get too close to the home continent, or the waters around it, become struck by the spores of this disease and get sick.

High altitude jet streams.

Your dragons fly at high altitude, and the jet streams are found only there. The jet streams eased their spreading out of the continent, but now travelling against them is too much effort for the dragons, thus they cannot go back.

The ships instead, dealing with surface winds, have not such limitations.

It is possible that the dragons have simply been away from the continent for so long that evolution has made it impossible to go back. On earth, many island-dwelling bird species have this same issue, where their ancestors came over to the island from the mainland and adapted to the new climate, but in the process deviated so much from the original species that later generations were no longer compatible with the original environment. This could easily be done in your story by specifying a difference in the climates of the two continents. Considering dragons are reptiles (sort of), your best bet would be with temperature or humidity, both of which mess with a reptile's ability to regulate its body heat. So, with this method, the vast majority of dragons would be unable to go back to their continent of origin, but there would still be individuals that may have a better tolerance for the old environment than the others, which could lead to some interesting plot developments.

Dragons glide, they can't fly on their own power. As long as they can find warm, upwards air currents they can stay up. But on the ocean? Eventually they fall and become kraken food.

That, or use a f... Maelstrom of air that ships can go around. Make it natural and non-magical. The way around the permanent storm is longer than a thousand miles.