It may be overgrown. I would be tempted to dig it up, divide and replant in well prepared soil.

yes it could need dividing up that seems to promote growth in old plantings,,,perhaps though it is just a young crown.
lawm clippings sure pump it up for next year,,,and lots of any kind rotted manure,,,best of luck

I have 2 year old rhubarb that does that when we have a dry spell and the soil dries out they flop like that & don't thicken up. Then new stalks grow from the center, so I harvest the outer ones to encourage more new stalks. Mine have plenty of composted manures and mulch, but probably need more consistent moisture and sunshine than they currently have. Also need more time to mature as they were small divisions planted in fall 2009.

Maybe it is just my monitor, but that doesn't look like rhubarb to me. LOL

Your rhubarb appears to be in a bed with other things around it plus a large paving stone up against it. There is Solomons Seal alongside which implies it is in a shady spot too. I think it would be happier in a lighter, moister, dedicated space of its own. It will need lots of rich organic matter (compost, manure etc) and plentiful water. Another possibility it that it is seedling rhubarb and not a named variety. I am afraid I can only sympathise rather than empathize. We have had so much rain that my rhubarb is almost as big as my wrist. The stalk in the sink at the moment is 26 inches long and weighs 15 ounces.

Have you fed and watered them?

Rhubarb is tough if it is a very old plant but even then they cannot simply be ignored if one wants them to produce to the utmost degree.