It is the wrong texture

Check the thermometer first

Before changing anything about the recipe, verify the thermometer in boiling water and compare it with what boiling water should read at your altitude. An instrument a few degrees out explains the symptom completely and is the cheapest thing to rule out.

Then carry-over heat

A heavy pan keeps cooking the syrup after the heat is off, sometimes by several degrees. If the number was right at the moment you read it and the product is still overcooked, this is where it went, and the fix is to pull earlier rather than to change the target.

Then altitude, if the recipe is not yours

Every stage temperature shifts down with elevation, so a sea-level recipe followed literally at altitude overcooks. This catches people once and then never again, and it is worth ruling out before suspecting the recipe.

Then the formulation

The proportion of corn syrup, butter, cream or acid changes the final texture independently of the cook, and a substitution made for cost or label reasons will move it. A change in supplier for one ingredient counts as a substitution.

Humidity shifts it slightly too

A batch made on a humid day finishes with marginally more water and reads softer. It is a smaller effect than the others and it is enough to explain a batch that is almost right.

Change one thing at a time

The temptation is to adjust the temperature, the formula and the technique together, which teaches you nothing. One variable per batch is slower and is the only way to end up knowing what happened.

Keep a correct piece to compare against

Texture is hard to judge from memory and easy to judge side by side. A retained piece from a batch that was right turns a vague sense that something is off into a specific observation, which is what you need before changing anything.

References