The difficulty in observing the separation of cis- and trans-2-butene by TLC might arise due to the close similarity in their physical properties such as boiling points, solubility, and polarity. These similarities can lead to overlapping or indistinct spots on the TLC plate, making it difficult to distinguish between the two isomers. Additionally, the separation of the isomers on TLC is highly dependent on the solvent used, the type of TLC plate, and the developing conditions, which can further complicate the observation of the separation. Hence, careful selection of the solvent, TLC plate, and developing conditions, as well as the optimization of the separation conditions, are important to obtain clear separation of the two isomers by TLC.