First, make sure impurities are off the mixture, make sure again! All solvents you use for all purposes with this product SHOULD be anhydrous, ANHYDROUS! Try trituration and/or solvent crystallization especially dry ether with seeding (if have prior standard) or scratch for 1-2 HOURS continuously with dry (preferable chilled) ether in it! Give it a try, Dear!

Article A Newcomer′s Guide to Peptide Crystallography

https://www.researchgate.net/post/How_to_get_or_crystallize_solid_amino_acids_derivatives_and_peptides

https://www.wikihow.com/Crystallize-Organic-Compounds

I worked as a bench chemist for over forty years and the one thing I can say is that recrystalization is an art that seems now to be an old school technique. Finding the correct solvent or solvent combination is empirical and is based on accumulated experience. A knowledge of solvent polarity is useful.

There can be several reasons that your product is an oil. But assuming that it could be a solid when pure there are a number of things that can be checked. I assume that the TLC is clean but if minor impurities are seen this could keep a compound from crystalizing. If impurities are seen then chromatography is suggested. If no impurities then the first thing to be done is to put the oil under high vac for several hours to remove trace solvent. If no solid then run it through a column as trace TMS impurities may be present. If the compound solidifies after this it still should be recrystalized. Good idea to save some seed crystals. A tight melting point is a good indicator of purity. Always check the supernate by TLC.

Having said all that it appears that your compound has at least one chiral center.

If your compound has two chiral centers, which will be differentiated by TLC, you may have to separate the diasteromers by chromatography. How do you maximize optical purity for each?

What would you do if your solid compound had a high vapor pressure under vacuum?