If your dressing has firmed up, it is likely due to the emulsification process. Emulsification is the process of combining two liquids that do not naturally mix, like oil and vinegar, into a stable mixture. The emulsification can break or separate over time due to various factors such as temperature changes, prolonged storage, or improper mixing, causing the dressing to firm up or become thicker. To restore the consistency of the dressing, try whisking or shaking it vigorously to re-emulsify the ingredients. If the dressing doesn't come back together, slowly add a small amount of vinegar while whisking to help re-establish the emulsion.