100 mg is actually a really low dose of labetalol that drs start with. You can go a LOT higher. I've had my dose adjusted upwards five or six times on this pregnancy so far. So it might be that as things continue, you need to adjust the dose for best effect.

I was wondering if anyone has found labetalol to have little effect?

I was given a prescription for 100mg BD at 17 weeks when my BP was 180/106. After a week or so my BP came down to 120s/80's but looking back I wonder if it was the second tri dip rather than the meds. I am now 22 weeks and over the last few days my BP is now consistently sitting in the 140-150/80-90. I know it's still not super high but it's the same on waking, a couple of hours after taking meds and it only goes up after activity but never down. I am aiming to keep my bp below 140/90 but above 110/70 to perfuse the placenta properly. Just wondering what other people's experiences were? I found this in my last pregnancy and I wonder whether pregnancy just resets my baseline bp.