I have to say that reading your post and history sure made my blood boil. You've had two canceled cycles due to thin lining and your clinic sure is taking a laid-back attitude. Have you done anything different from the cancelled cycles this time to improve lining thickness?
I give the following advice to all gals who've had trouble with thin lining: the optimal chance of am embryo implanting is when the lining is between 9 and 14mms. Anything below or above that and chances of implantation and live birth decrease. The research to date is very clear on this.
Sadly, it is a common practice for North American clinics to set the cut-off for cancelling a cycle at anything below 7mm. It's not that there is zero chance of implantation at 7, but likelihood is reduced. The clinics abhor cancelling cycles as this reflects poorly in their reported statistics (success rates include all STARTED cycles, regrdless of cancellation). They would rather take the chance that the embryo will implant at 7mm (since SOME will) than cancel the cycle and reduce their success rate. Also, patients are never satisfied when a cycle is cancelled, so they set the bar for cancellation lower than the optimal measurement for higher rates of implantation. Sad but true.
So my advice is NEVER to transfer below 9mms. You risk wasting perfectly good embryos. You have control over whether to transfer or not, even though your clinic will push back. Stay strong!
For your current cycle, how many days into your cycle are you? If you are more than 10 days in and you are at only 6mms, then you should speak to your RE about doing something different now. Otherwise, you risk not getting to at least 9mms.
If your clinic brushes you off, consider another clinic or at least another RE. With two cancelled cycles under your belt, they should be doing things differently instead of more of the same.
Wishing you tons of luck. And btw, my history is very similar to yours. I only had success when I transferred with a lining above 9mm. Gave us our little girl.
What works for one woman may not work for another...I always had trouble getting a thick lining....and my doctor put in 3 5 day blasts when my lining was 6.7 and I had identical twins....and I think only once I had a lining over 8 mm...and that was a bust....so go with your gut and if the embryo is strong enough they will find room to "dig in".....