A young Monstera adansonii or Monstera deliciosa almost always starts with solid, unsplit leaves. That's normal, not a sign of a sick plant. The first fenestrations rarely arrive as a full set of splits either. On a deliciosa they tend to show up as a single notch cut into the edge of an otherwise solid leaf, and on an adansonii as a couple of small holes near the midrib. One notched leaf means the plant has started, and the leaves after it usually go further. Split leaves — fenestration — are a response to two things: maturity and light. A cutting has to grow past its juvenile phase before it can produce fenestrated growth at all, and no amount of fertilizer speeds that up. Most indoor monsteras need several new leaves, sometimes a year or more of growth, before splitting begins. Light is the lever you actually control. In the wild, monsteras climb toward brighter gaps in the canopy, and splitting is thought to help light reach lower leaves and let wind pass through without tearing the plant apart. Indoors, a monstera kept in low or medium light will often keep producing solid leaves indefinitely, no matter how old it is. Move it somewhere with bright, indirect light — close to an east or west window, or a few feet back from a south-facing one — and new growth usually starts showing splits within a few leaves. Leaf size is the reading to take while you wait. On a plant with enough light, each new leaf comes in a little larger than the one before it. Leaves arriving the same size, or smaller, say the light is short before anything else does, and a plant working its way down in leaf size will not begin splitting however long it is left. A support pole helps too. Monsteras are vining araceae; a plant allowed to climb tends to mature and fenestrate faster than one left to sprawl. A pole only helps if the stem is against it. Aerial roots have to reach the surface and grip, so tie or clip new growth to the pole as it extends rather than leaving it to lean away. Once those roots have taken hold, leave them alone - cutting them off removes the thing the plant climbs with. What won't help: more water, more fertilizer, or repotting. Overwatering in particular just risks root rot while you wait. If a plant is getting strong indirect light, has something to climb, and is still putting out entirely solid leaves after a year, patience is still the right answer before intervention.
Why Your Monstera Hasn't Split Yet
Fenestration is a light problem before it is an age problem.