Yes, I realize that it isn’t enough sun. It’s the sunniest spot I have available. It’s near my roses and other full sun plants. I’ve watered it nearly every day that we haven’t had rain. It may just be a little guy. Can I give it fertilizer? I don’t grow anything but herbs here, so I’m very inexperienced this kind of stuff.
Upthread Diana mentioned 20-0-0 for broccoli for big heads, but I don’t know about broccolini. We just grow the broccoli and our soil is high in nitrogen so it does well here. We use compost so enriching the soil is always good. I think your biggest factor is the sunlight. The plant looks very healthy. So I would not expect too much from the one plant. I have limited sun in parts of the yard but the garden is OK. I track the sun every few years from spring until fall during the day to find the sunniest spots for flowers. Maybe broccolini does well in a large container if that could go somewhere with more sun. You would have to have a rich soil mix.