Dear Mahdie,

thanks for posting this interesting question on RG. In general, when a compound is insoluble in a particular solvent, then it is insoluble. Of course, this is normally specified for normal condtions, i.e. normal pressure and room temperature. As you certainly know, solubility can significantly increase upon heating. Sonication in an ultrasound bath is also often helpful in obtaining clear solutions. However, If a material has been studied at different temperatures and found to be insoluble in a solvent, than it simply cannot be dissolved in that solvent. In the case of well-defined compounds, alkyl substituents can make the compound more soluble (as already indicated by Pedro).

Good luck with your work!