Acer isn’t unfurling. Any ideas?

Despite a promising start my Acer seems to have completely halted the process of pushing out its leaves. It’s been like this at least two-three weeks and I’ve not noticed any changes in that time.

During the sunny period I kept it sheltered and in partial shade, and gave it a water every few days. I did notice some green aphids (?) at the beginning of April but I gave them a gentle spray with an insect repellent I’ve used on my other plants before.

Would anyone have any suggestions as to why it hasn’t fully opened its leaves?

Thanks!