Sonication to purify solid product- the crude product just disappeared I am forming Fmoc-valine-citrulline dipeptide, and below is the literature protocol I am following (from Bioconjugate Chem 2002, 13 (4), 855-869):

Formation of Fmoc-Val-Cit


Fmoc-Val-OSuc (succinimidyl ester) ($\pu{14.91 mmol}$) in DME ($\pu{40 mL}$) was added to a solution of Cit ($\pu{2.743 g}$, $\pu{1.05 equiv.}$) and NaHCO3 ($\pu{1.315 g}$, $\pu{1.05 equiv.}$) in water ($\pu{40 mL}$). THF ($\pu{20 mL}$) was added to aid solubility, and the mixture was stirred at room temperature for 16 h. Aqueous citric acid (15%, $\pu{75 mL}$) was added, and the mixture was extracted with 10% 2-propanol/ethyl acetate ($\pu{2 \times 100 mL}$). The solid product began to precipitate but remained in the organic layer. The suspension was washed with water ($\pu{2 \times 150 mL}$), and the solvents were evaporated. The resulting white solid was dried in vacuo for 5 h and then treated with ether ($\pu{80 mL}$). After brief sonication and trituration, the white solid product was collected by filtration

I confirmed the formation of the crude product through HRMS and NMR. I then proceeded to carry out sonication to purify. I thought the purpose of sonication was to initiate crystallization. However, when I sonicated the crude product (~20 mins), I observed that the product was well-dissolved in ether, rather than forming a crystal. I used a Büchner funnel to collect the product but there was nothing left on the filter paper. Would pouring more slowly over the filter paper solve the problem? And more importantly, I want to understand the purpose of sonication step.
So I think we have established the problem (thank you @user6376297) . The material still contained enough iPrOH to cause it to dissolve in ether rather than give a filtrable solid because it was not adequately dried. What you need to do with your ethereal solution of product is remove the solvent under reduced pressure then dry the resulting solid under vacuum for at least 5hrs and preferably overnight. Then briefly (~1 min) sonicate the dried solid with ether and collect by filtration.
There is a lesson here: if you have a literature procedure that delivers your desired product in good yield then you follow it - without shortcuts.