Wash the whole post-reaction mixture with vigorous shaking with 1/3 its volume of water, followed by 1/3 its volume of conc. sodium chloride, and if it doesn't crystallize already in the sepfunnel, drain it into a beaker, scratch the inside of the beaker with a glass rod, and then recrystallize it from methanol or isopropanol.
If all else fails, make a seed crystal of phenyl-2-nitropropene using this method: https://www.thevespiary.org/rhodium/Rhodium/chemistry/edda.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/edda.html)
Then add such a crystal to your liquid phenyl-2-nitropropene - it will crystallize immediately. Always save a seed crystal for future use of all hard-to-crystallize compounds, you will never regret it!