Some of the Rockies weaknesses should not surprising.
#2 weakness: First base
Of all the positions, this position has seen the biggest drop-off in fWAR rankings. Entering August 9th, the Rockies fWAR at first base was tied for 1st in baseball at 0.5. Since then, Rockies first baseman have a fWAR of -0.5, which is 27th in baseball in that span.
That is mainly because the dropoff that Daniel Murphy has had. Through August 8th, he was hitting .372/.413/.628 and since then, he is hitting .176/.208/.191 with zero home runs and four RBI in 68 at-bats.
This is why Josh Fuentes has received six starts at first base in the last two weeks. From August 25 (his first time in the starting lineup in 2020) through September 6 (the last time he was in the starting lineup to date), he hit .385/.414/.615 (10-for-26) with one home run, 6 RBI, and three doubles.
Honorable (or, in this case, dishonorable) mentions: The starting pitching and centerfield
Since August 9th, the Rockies starting pitching and center fielders have been in the bottom third of baseball as well. The Rockies pitching staff was 4th in fWAR entering August 9th (2.0) but they are tied for 20th since (1.0), entering Wednesday.
Center field has long been a weakness for the Rockies and it has been for the entirety of 2020, partially due to the lack of production and, later, the trip to the IL for David Dahl. Before August 9th, Rockies center fielders were tied for 21st with a -0.1 fWAR and they are 22nd since with a 0.2 fWAR.