Without seeing the NMR data it is difficult to say, but NMR is always likely to spot things that TLC can't.

Try running TLC with a different solvent system. Occasionally, compounds elute in the same fashion using a given stationary phase and solvent combination. Try using changing the solvents for example- from hexane/ethyl acetate to hexane/ether; hexane/toluene, hexane/dichloromethane, or hexane/ethanol.

The second solvent (strong solvent) in each of those are from a different selectivity group causing compounds to elect at different relative retention times. Reverse phase TLC is possible but often takes a long time- but reverse phase HPLC is faster and may resolve the compounds.

Hi Tey,

Perhaps because your product still contains impurities. Therefore, you need to purify your product. If tlc cannot improve the purity of your product, you can use other purification methods such as recrystallization, distillation, or use instruments such as gc-ms.