Marginal distribution growth over the past few years relative to the growth they exhibited during their early years.
That and the looming fact that Linn (or Linnco) needs to issue over $1 billion in equity to right the balance sheet. That and the fact that their natural gas realized pries have been falling for several years, putting pressure on their coverage ratio and necessitating the large acquisitions. They also are feeling a small pinch from NGL prices dropping moderately which hurts them given their lack of NGL hedges. All of these factors are contributing to the stagnation in equity price. Also remember, they have 232 million units (and shares) outstanding. It becomes harder and harder to grow on that base each year. A tiny 5% distribution boost requires over $33 million in accretive DCF. This requires them to make $1-$3 billion dollar deals annually. Those are much rarer and not always a good fit in an MLP.
I agree that LINE's size necessitates big acquisitions and thus big SO's to make much of an impact on DCF . That's why I tend to favor small cap MLP's, they are more likely to perform like LINE did when it was small. LINE's model of stable distribution/yield through hedging also exposes it to interest rate risk,somewhat similar to a bond- as interest rates rise, LINE's fixed yield will lose appeal and would likely put downward pressure on unit price