The S&P/ASX 300 (Index: ^AXKO) (ASX: XKO) is up 0.8% at 5,350.8 points in afternoon trading – and roughly the same gain over the past week.

Today’s rise was likely due to gains on US markets overnight where the US central bank held interest rates steady as most economists had predicted.

That hasn’t helped these four companies, which saw their share prices sink…

Pilbara Minerals Ltd (ASX: PLS) share price dropped 7.9% to $0.47. Investors may be selling out of the lithium developer after the company appeared to breach an agreement it has with Mineral Resources Limited (ASX: MIN) about the sale of lithium products from the Pilgangoora project. Mineral Resources apparently has the right to match any deal for the sale of lithium concentrate produced from Pilbara Minerals’ project and is claiming Pilbara has breached that. Pilbara’s share price is down 15% in the past week.

OFX Group Ltd (ASX: OFX) share price dropped 6.4% to $1.98 and is now down more than 18% since a month ago. The foreign exchange payments provider hasn’t released any news in the past month – which could explain why news-hungry investors have lost patience and are selling out. In a recent article by colleague Matt Brazier, he highlighted a number of issues that OFX Group appear to be experiencing, and perhaps investors have started to take note.

Spark New Zealand Ltd (ASX: SPK) share price dropped 4.5% to $3.38. Spark is the retail equivalent of Telstra Corporation Ltd (ASX: TLS) in New Zealand, although it has a much tougher competitor in Vodafone in NZ than Telstra does from Vodafone here in Australia – with the two Kiwi companies reporting very similar levels of revenues. Today Spark announced the release of an entry-level broadband service – Spark Jump – in NZ for only NZ$15 a month, pre-paid and with no contracts. Investors could view that as potentially cannibalising its higher cost plans, cutting revenues and margins – hence the sell-off.

JB Hi-Fi Limited (ASX: JBH) share price dropped 3.1% to $28.76. The consumer electronics and whitegoods retailer is in the process of acquiring The Good Guys at a cost of $870 million. The deal announcement initially saw JB Hi-Fi’s share price soar above $30, but shareholders have been asked to kick in around $394 million buying shares at a price of $26.20 each. Some shareholders may be taking advantage of the arbitrage opportunity – selling their shares on market and they applying to buy more under the retail entitlement offer.

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Motley Fool writer/analyst Mike King doesn't own shares in any companies mentioned. You can follow Mike on Twitter @TMFKinga

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.