@Dhano21 This caused me no end of anxiety from week 16 - onward! My advice to you is: try not to read too much into what others have said RE: movements. We really are all different, and so is your baby.
I, like you, have a posterior placenta and felt my first flutters quite early on. It was around 16-weeks I felt my first kick, but that's only because I was leaning back showing my DH how my uterus looked like a pear and the baby happened to kick at the precise moment my hand was in the exact place. Otherwise, I'd have never felt it. It was weeks until I felt another.
After that, it was not regular at all. I felt no more kicks or big movements until well after 26 weeks. I panicked a lot during this time, because people would tell me what's "normal" according to Dr. Google, or whatever the NHS "standard" is. I would convince myself something was wrong because I should have felt the baby move more by now.
Please please please know, there is no "standard" there is just averages based on whatever data they have. I've been in Triage 3 times with reduced movements, but really I just have a lazy and quiet baby. As soon as I'm hooked up to the monitor the heartbeat is present and strong.
It's only really daily movements I feel the last few weeks, and that has been perfectly normal for my baby. Some women carry very active children, others (like myself) carry babies who find a spot they like and don't really move much from it.
I'm currently 37 weeks and still only really feel baby having a wee stretch of its back and doing something weird with its hands (that feels like it's churning butter or something in there).
If you are at all worried at any point, just pop along to your day unit or triage, they'll happily hook you up for 20-mins to check baby is ok. I also got a home doppler to ease my anxiety (other posters, don't @ me about the doppler) and it helped a lot. You must always trust your gut instinct though, if you think something is off based on your experience (not others') then get checked out.
It's so much easier said than done, but try not to worry too much :-) (coming from someone who worried like crazy for most of the 2nd and 3rd trimester).