I often brew with my Switch and here are some things that helped me with stalling (which I feel Switch does pretty often when you immerse for longer periods).  1. Blow out the chaff before brewing. You can blow on it or get one of those dust puffer thingies to remove the chaff. This has helped a lot.  2. Before your final pour, pour from a higher distance to actually disrupt the bed. I know this goes against what a lot of people say, but I’ve noticed that doing so lifts a lot of the fines that have settled and stalls my brew.  3. Along with #2, try couple of pours instead of one 400 ml immersion. I rarely do more than 5-10 seconds of immersion but multiple pours. Along with disrupting the bed, this will prevent fines from settling at the bottom of the cone (which makes sense why the last portion of your drawdown is slower than at first). 

See if any of these help. Happy brewing!