There has been a new trend lately of the Chinese radio manufacturers making DMR radios that don't do TDMA. TDMA is the pulsing the radio does that actually makes it transmit only half the time on the channel, allowing another user to transmit in the other half. This is what allows DMR repeaters to repeat two conversations at once. TDMA is a fundamental part of the DMR standard.
These new "DMR" radios that have only come about recently are unable to do TDMA. They are being sold in many cases as tier 2 DMR radios, which means they should be able to do TDMA to use repeaters properly, but they cannot. (See my video for an explanation of the difference between the tiers).