I remember hearing one of those long form group interview type programmes years ago. The group was former whips from all the main parties and form different governing periods.
A lot of the discussion was about the scurrilous tricks whips might use but the one point all of them agreed on was that it was easier to whip a small majority than a large majority. Not just due to the numbers to whip but due to the lack of fear in large majorities. The view was that MPs in a small majority are in constant fear of an early election which could cost them seats or their government posts (with benefits) whilst MPs in a large majority simply don’t believe they are at risk for several years.
Since then I’ve heard echoes of the same sentiment from other whips over the years so I imagine this is the problem the current Labour whips are experiencing.