From Wikipedia:
According to the most popular version of the singularity hypothesis, called intelligence explosion, an upgradable intelligent agent will eventually enter a "runaway reaction" of self-improvement cycles, each new and more intelligent generation appearing more and more rapidly, causing an "explosion" in intelligence and resulting in a powerful superintelligence that qualitatively far surpasses all human intelligence.
But what if the complexity of the problem of self-improving the software grows at a faster rate than the AGI intelligence self-improvement?
From experience we know that problems tend to be harder to solve at every iteration, with diminishing returns. Take as an example the theory of gravitation. Newtonian physics is relatively easy to formulate and covers the majority of high level gravitation phenomenas. A more refined picture, like General Relativity, fill few holes in the theory with a huge increase in complexity. To describe black holes and primordial cosmology we need a theory of quantum gravity, which appears to require a further step in complexity.
What "saved" us so far is the economic growth of our civilisation, which allowed more and more scientist to focus on solving the next problems. It's true that the first AGI will have the luxury of being duplicated, being a software base intelligence, but at the same time is likely that the first AGI will be extremely compute intensive. But even assuming that we (or maybe it would better to say, they) have the hardware to run $10^{2}$ instances, if the complexity of every substantial self-improvement grows say by $10^{d}$x with $d=3$ while the improvement to the intelligence is only $10^{l}$x with $l=1$, the self-improvement cycle will quickly slow down.
So is increasing software complexity the most likely bottleneck to the AI singularity? And what are likely values for $d$ and $l$?