Today was a slightly down day for the S&P/ASX 200 (INDEXASX: ^AXJO) (ASX: XJO), which lost 0.51% to 5,252 points.

A number of shares fell substantially further however, and here’s why:

Sky Network Television Ltd (ASX: SKT) plunged 16% to $4.28 after the company announced a loss of 45,000 pay TV subscribers in New Zealand, likely a casualty of ongoing customer uptake of streaming video and movie services like Netflix. Sky has previously told the market that its future operations would be characterised by higher churn and lower Average Revenue Per User, as well as lower capital outlay, lower content cost, and lighter customer service demands. With Sky clearly still feeling the pinch of competition, it’s tough to argue shares are a buy at today’s price – especially with some of its services still more expensive than Netflix’s comparable offering.

Sky Network shares are down 28% in the past 12 months.

Cover-More Group Ltd (ASX: CVO) fell 4.5% to $1.38 in a roller coaster week where shares were sold down heavily on a market update followed by a subsequent presentation that restored much of investors’ battered confidence. The market is still apparently uncertain how to value Cover-More in light of slowing Australian growth alongside significant potential opportunities overseas. The company’s partnership with Flight Centre Travel Group Ltd (ASX: FLT) ought to serve both businesses well in the long term, although I have found that Cover-More’s insurance is considerably more expensive than competitors’ offerings.

Cover-More shares are down 35% in the past 12 months.

Medibank Private Ltd (ASX:MPL) lost 6% to $3.10 today in potential profit taking, after the company hit a new all-time high of $3.28 just yesterday. A presentation this morning may have also cast more light on the company and its prospects for some investors. At these prices, Medibank is trading at a lofty ~24 times estimated full-year earnings, which is quite expensive for an insurer – even one with favourable long-term tailwinds and a cost-cutting program. I wrote just this morning that although Medibank is a strong business, buyers are not getting a reasonable margin of safety in purchasing shares today.

Medibank shares are up 43% in the past 12 months.

BT Investment Management Ltd (ASX: BTT) dived 7% to $9.46 despite releasing a strongly positive report yesterday that saw cash profits rise 26%, while margins, dividends, and funds under management also improved. Shares closed slightly down yesterday, and today’s sell-off could be due to management’s tacit acknowledgement that they had no way of predicting the outcome of a potential ‘Brexit’, even though BT was in a ‘strong’ position to weather possible impacts. Recent results look strong despite the tailwind of a lower Australian dollar, and investors could also be worried that a lower British pound would hurt returns from BT’s strong UK segment in the future.

BT Investment shares are effectively flat for the year, down 0.7% at the time of writing.

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Motley Fool contributor Sean O'Neill owns shares of Flight Centre Travel Group Limited. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.