Without having a specific curtain tool, I actually found the 5" CFR is the best tool for curtains. Tried the Sapphire and the DM2 and they are both shyte on curtains.
Your problem is wetting the fabric out evenly but not overwetting. The DM2 isn't very ergonomic for curtain work, but more importantly it just massively overwets and spills out the sides..... unless you turn it right down, in which case it simply doesn't work.
The sapphire p*s out the edges as well, and if you turn it right down it just leaves pin-stripe lines which to my logic says it simply isn't flushing the fabric through.