Sorry to hear you're having problems after being so fertile with your first few I can truely understand your frustration. I fell preg with my DS on the third month ttc and am now 8 weeks pregnant after 2-2.5 years TTC....It was a rough trot as I easily fell preg before we were told to keep trying and numerous tests showed nothing was wrong. After a chem preg then 2 x IUI, 1 x IVF and 2 x FET all BFN I got tested for a myriad of things and they found my natural killer cells were over activated (ie my immune system is on overdrive), treatment for this and a FET and I got my BFP first time.
I think if it was 'easy' before and not much has changed then i would ask for testing, it could just be that you've been luckier in the past but if there is something it's good to know so you can have a plan. Good Luck xx