My recommendation is to stir your acid with HBTU for 5-15 minutes before adding your amine. The amine can be added drop wise via a syringe. This did the job for me with 90+% yields for most substrates. The choice of the solvent is very cruical as well.

Thanks a lot, I will use DMF hopefully of a good grade

In addition to what Ahmed shared above, I recommend to use N-ethyl-N,N-diisopropylamine (EDA, Hunig's Base), at least 1 eq. First, the acid in an appropriate solvent is treated with EDA. The process is then conducted as Ahmed recommended. In our lab, we successfully use acetonitrile as a solvent for both couplings in solution and to amines attached to solid phases.

Thanks, we can try ACN as well