The S&P/ASX 200 Index (ASX: XJO) has run out of steam and slipped into the red on Wednesday. At the time of writing, the benchmark index is down 0.3% to 8,292.7 points.

Four ASX shares that are falling more than most today are listed below. Here's why they are dropping:

Bapcor Ltd (ASX: BAP)

The Bapcor share price is down 3% to $5.04. This morning, this auto parts retailer released a trading update ahead of its annual general meeting. Bapcor revealed that its total revenue during the first quarter was up 0.7% over the prior corresponding period. This is a major slowdown from the 7.7% growth it reported during the first five weeks of the financial year. Management advised that Bapcor's Australian trade and Specialist Networks businesses are performing well. However, the retail environment, and its performance in it, continues to be more challenged.

Boss Energy Ltd (ASX: BOE)

The Boss Energy share price is down 4.5% to $3.41. This follows the release of an exploration update from the uranium miner this morning. Management notes that the drilling, which was designed to expand the producing wellfield capacity, continues to significantly exceed the cut-off grade thickness requirements for In-Situ Recovery (ISR) of uranium. However, it seems that some investors were expecting even stronger results.

Challenger Ltd (ASX: CGF)

The Challenger share price is down 4% to $6.55. Investors have been selling this annuities company's shares following the release of its first quarter update. Challenger revealed total annuity sales of $2.4 billion for the period. This reflects strong sales in retail lifetime annuities, Japanese annuities, and Challenger Index Plus, and lower fixed term annuity sales. Management also revealed that group assets under management rose only 1% for the quarter. That's despite the market rising significantly more.

Rio Tinto Ltd (ASX: RIO)

The Rio Tinto share price is down 1% to $121.00. This has been driven by the release of a third quarter update from the mining giant that fell short of expectations. For the three months ended 30 September, Rio Tinto reported Pilbara iron ore shipments of 84.5Mt. This represents a 1% increase year on year and a 5% lift over the second quarter. However, the consensus estimate was for shipments of 85.1Mt. The miner also fell short of expectations for aluminium and copper. Finally, management advised that Pilbara iron ore unit cash costs are now expected to be at the upper half of its US$21.75 to US$23.50 per tonne guidance. This is due to inflation being at the higher end of its expectations.