Progesterone problems!! Please help!

I have been on progesterone in oil shots since a 6 week blood draw showed my progesterone level was at 7.6 after a fresh successful transfer with normal hcg levels and rising and doubling like they should...the doctors then took me off at 13 weeks and told me everything should be fine because the placenta takes over. Well 2-3 days after stopping I experienced some cramping and then brown discharge since I was released to my OB I called and requested a blood draw just to check and make sure everything was going smoothly...well my level came back at 6.3 and it was 20, 5 days ago I’m now back in the shots but why is this happening has anyone ever experienced this? Is the placenta not working? They ordered a full scan and baby and placenta appear to be fine but my progesterone is not. I’m really just hoping to find anyone who has experienced this...am I looking at shots for the whole pregnancy?