Hi, i don't own a RD6 but i did have TD3 and i had a similar issue (but now it's working fine)
The td3 cannot react to MIDI input if it's playing so when i connect it to the BSP i need to press play on the BSP, then press pause on the TD3 (so the only active sequencer is the BSP)
Also if i play only with the TD3 i just deactivate Transport in Midi control center so i dont have to re-press pause every time on the TD3
Also did you follow those steps on the RD6 ?
SETTING SYNC and CLOCK TYPE :
Step keys 1, 2, 3, or 4 represent clock source INT, MIDI, USB, or TRIG respectively.
Step keys 5, 6, 7, or 8 represent clock rate 1PPS, 2PPQ, 24PPQ, or 48PPQ respectively.
1. Press Function.
2. Make sure no sequence is playing.
3. Press PATTERN GROUP and WRITE/NEXT at the same time to enter Sync setting mode.
4. Selector 1 to 8 LEDs will turn on, and the current clock source (1 to 4) and clock rate ( 5 to 8 ) will flash, you have about 3 seconds to make any changes.
5. Press step keys 1, 2, 3, or 4 to change the clock source to INT, MIDI, USB, or TRIG respectively.
6. Press step keys 5, 6, 7, or 8 to change the clock rate to 1PPS, 2PPQ, 24PPQ, or 48PPQ respectively.
7. Press any other buttons or after no changes for a 3 seconds, the changes are saved.
8. Repeat step 3 to check your selection.
MIDI CHANNEL SELECTION :
Press and hold FUNCTION, then press PATTERN GROUP to enter MIDI channel setting mode, then press STEP KEYS 1-16 to select the channel, press PATTERN GROUP to switch between MIDI transmit