First polytunnel planting

[Peter B]

I put up a 15ft x 8ft polytunnel in February and have it fully planted up now. I'm new to polytunnel growing and though I've followed all the advice I can find in numerous books, I'm still unsure about several things, so I thought I'd sign up here and post my first 'problem'.

I'm in the far South West of Cornwall and have always had pretty good success with sweetcorn outdoors, but this year I thought I'd plant a square of 9 plants in the tunnel. They are an F1 hybrid seed which I brought on in the house, then planted them out in the tunnel when they were about 4 inches tall. I dug some well rotted compost into each hole and put just a sprinkle of bone meal in there as well. They romped away at first but then some of them faltered. One row of three, the nearest in the picture, are stunted and have thrown second main stems which I've never seen before, and some of them, even the strong ones, are acquiring yellowish leaves, as shown.

I should be grateful for any advice as to what's going on here because I don't really know what to do about it.

Thanks.