A well-made rasgulla compresses under a spoon and springs back. That behavior is not a matter of freshness or sweetness but of how the protein inside was built and then expanded.
Kneading builds a continuous protein mass
Drained chhena is a collection of separate curd grains. Left that way it will crumble apart in liquid, so it is worked by hand until the grains smear together.
Kneading uses friction and the fat already present to bridge the grains. The mass turns from crumbly to slightly tacky, which is the visual signal that the network is continuous.
Under-kneaded chhena disintegrates in the pot. Over-kneaded chhena warms too much, loses fat to the palm and turns dense, which shows up later as a hard center.
Boiling expands the ball from the inside
The balls go into thin, actively boiling syrup rather than thick syrup. Thin syrup is mostly water, and water moves into the protein mass readily.
As it does, the ball swells noticeably, often to well over its raw size. The protein network stretches around the incoming liquid and sets in the expanded position.
Thick syrup reverses this. Sugar concentration outside the ball pulls moisture out instead, so the sweet shrinks, hardens and never develops any bounce at all.
Sustained heat is what fixes the shape
If the syrup drops off the boil, expansion stalls partway and the protein sets around a partly collapsed structure. The result looks fine and eats heavy.
Keeping the pot at a rolling boil, with a lid to hold temperature when balls are added, is the most common instruction in these recipes and the most commonly ignored one.
Adding a splash of hot water during boiling keeps the syrup from thickening as it evaporates. That is maintenance of the expansion condition, not an adjustment of sweetness.
Cooling determines whether the spring survives
A rasgulla lifted straight out of hot syrup deflates as steam inside condenses. Left to cool in its own syrup, it holds the expanded shape and absorbs sweetness gradually.
This is also why they are served cold and sitting in liquid rather than plated dry. The syrup is structural, not garnish, and draining them early undoes the work.
Every failure traces back to one of three points
A flat rasgulla was under-kneaded or boiled too gently. A hard one met syrup that was too thick, or chhena that was worked for too long before shaping.
A cracked one usually had chhena that was too dry, which happens when it is pressed under weight for longer than the recipe intends and loses moisture it needed.