Then there is your answer, if taken sublingually/oral, serum levels may not be showing high, but you body is definately, or should be, putting that estrogen to use. 6 months is way early in a transition. If you feel fine, and are getting results, dont worry.
If e levels are higher then T, on an oral sublingually administered dose, things *should* be moving along.
How are your emotional peaks at? You feel E drop or no?
Also, if sublingual I assume that you take more then one dose daily split up to limit peaks/troughs?
Either way, and as aleays, if you have concerns bring them up with your medical professional.
Serum orally and sublingual tends to be a bit lower then intra muscular, or dermal applications. As long as it is doing the job, do not fret too much over numbers. Some bodies do pretty well low spectrum, some higher. Everyone is different.