IB these days hassles you if you trade an unspecified % of volume as if you might be an affiliate of the company and locks up your position, including from closing it, for some compliance review. Trading or acquiring moderate positions in low volume OTC stocks is probably best done elsewhere.
Capital Charges are big for over 10%. Most firms are not well capitalized to handle large volume penny OTC traders.
Capital Charges to the Brokerage from the Clearing House. Going over the thresholds for OTC liquidity means you have to post excess collateral temporarily to them. One account trading 100k of an OTC stock repeatly may cause firms to post Millions in collateral. They receive the posted collateral back but still takes a capital hit temporily from the trader. Smart firms should keep excess in Treasuries and simply post that to the clearing house for these penny stock charges.
IB got stricter after they got the big Fine for micro-cap trading. Basically got bullied by the regulator into submission.
Brokers get the collateral charges themselves imposed on them. Some either post the capital to the Clearing house or restrict trading to their clients. 1 client aggressively trading OTC shares will tie up more regulatory capital than 5000 regular clients.
Just Imagine a client trading 1M worth of shares and the brokerage having to post 25M,50M,100M in collateral. Most brokerages will believe it's not an efficient use of capital and just restrict trading.