Fenestrations — the splits and holes — are decided inside the developing bud, before the leaf unfurls. Once a leaf has opened flat and solid, it stays that way permanently. No amount of light, feeding, or care afterward will add holes to a leaf that's already grown in without them. Every leaf you see right now reflects the conditions your plant had weeks or months ago — what you fix today shows up in the next leaf, not this one.
That reframes the whole question. You're not trying to repair anything visible on the plant — you're setting up conditions so the next bud that unfurls does so with splits. With that in mind, here's what actually determines whether that happens.
Not mature enough yet
This is the single most common reason, and it's also the one no amount of adjusting light or water will speed past. A Monstera typically needs to reach roughly two to three years of age, or a handful of leaves and real height, before it starts producing any fenestration at all — plants younger or smaller than that will grow entirely solid, heart-shaped leaves no matter how good the conditions are, because the plant simply hasn't reached the developmental stage where it starts investing in holes.
What to do: keep every other condition on this list optimized so the plant reaches maturity as quickly as possible, but accept that patience is the actual variable here. A small, young plant with perfect care is still a small, young plant.
Insufficient light
Once a plant is old enough to fenestrate, this is the most common fixable cause when it still isn't. Fenestrations are believed to help a large leaf let dappled light reach the growth below it in the wild — without enough light to justify that trade-off, the plant has less reason to invest in the more complex leaf structure and tends to default to smaller, solid leaves instead.
What to do: move it a few feet from an east or west-facing window, or in front of a south-facing one with a sheer curtain to diffuse direct sun. Give it a few new leaf cycles after the move — remember, the leaf unfurling right after you move the plant was already decided beforehand.
No climbing support
In the wild, Monstera climbs tree trunks, and plants grown vertically against a moss pole or similar support tend to produce larger, more fenestrated leaves than ones left to sprawl along the ground or trail from a shelf — mimicking that climbing habit seems to signal the plant to grow the larger, more mature leaf form.
What to do: give it something to climb — a moss pole, coir pole, or trellis — and loosely tie the stem to it as it grows, rather than letting it trail.
Water stress
Both consistently soggy soil and consistently dry soil put the plant under stress, and a stressed plant tends to prioritize basic survival over developing more complex, fenestrated leaves. Soggy soil risks root rot on top of that; dry soil pushes the plant to conserve energy with smaller growth generally.
What to do: water when the top few inches of soil are dry, in a well-draining aroid mix, rather than on a fixed schedule in either direction.
Nutrient deficiency
A Monstera that hasn't been fed in a long time, especially one already repotted into old, depleted soil, may put out smaller and less fenestrated leaves simply because it doesn't have the resources to invest in bigger, more complex growth.
What to do: feed with a diluted, balanced liquid fertilizer every four to six weeks during spring and summer, and hold off in autumn and winter when growth naturally slows.
Inherited immaturity
Where a cutting comes from matters more than most people expect. A cutting taken from a mature, already-fenestrating parent plant carries some of that developmental maturity with it and can start producing splits within months — while a cutting or seedling-grown plant with no fenestrating history starts from the same juvenile baseline as any other young plant, regardless of how big the parent eventually got.
What to do: if you're starting a new plant specifically for its fenestration, a cutting from a confirmed mature, fenestrating parent is a genuine shortcut. If you already have a young plant grown from seed or an immature cutting, the age requirement above still applies no matter what you do.
One more thing worth saying plainly
A Monstera without fenestration isn't sick, and it isn't a sign you're doing something wrong at a basic care level — plenty of young, perfectly healthy plants simply haven't gotten there yet. It's a cosmetic, developmental trait, not a health indicator, and there's no guarantee attached to any of the fixes above beyond improving the odds.