A solid Monstera leaf is not automatically a sign of poor health. Young plants often make entire leaves, and a new leaf may have only a few openings before later leaves become more divided. The photo shows a new Monstera leaf with one opening while it is still unfurling. It is an example of variation, not a promise that every next leaf will split.
Is the plant mature enough?
The Royal Horticultural Society notes that young Swiss cheese plants usually lack their characteristic holes. As a healthy plant matures, its leaves can become larger and more perforated. Check the species too: Monstera deliciosa and Monstera adansonii have different leaf sizes and habits. A cutting from a mature plant may also produce smaller leaves while it establishes.
Existing solid leaves will not develop new holes later. Watch the size and shape of new leaves over several growth cycles instead of judging one leaf in isolation.
Could it need more light?
The RHS says plants growing in poor light often have fewer holes and recommends bright, filtered light. Compare the new growth with the plant's position: is it several metres from a window, behind a shade, or making smaller, stretched leaves? Maryland Extension explains that light intensity falls with distance from a source.
Move the plant gradually to a brighter position that avoids harsh direct sun. A sudden move into strong sun can scorch leaves. Rotate the pot occasionally if growth leans toward one window. In a dim room, a suitable grow light may help, but follow the light's placement instructions and provide a dark period.
What else should you check?
Keep the roots healthy with a draining potting mix and water according to the plant and season. The RHS growing guide recommends letting the compost approach dryness before watering thoroughly and avoiding standing water. If growth has stalled, check roots and pests before adding fertilizer or moving the plant again.
Fenestration depends on maturity and growing conditions; no single light adjustment guarantees a split leaf. If new leaves are yellow, damaged, or much smaller, investigate watering, light, and roots as separate questions. A photo of one solid leaf cannot tell you the plant's age or light history.