Have you had any recent bloods done with your natural level of hcg? When testing you always seem to get these faint lines, which just seems odd that they don't develop further, they can't all be faulty tests. I wonder if either you are conceiving and then it is failing to implant properly or if your hcg levels are naturally elevated a bit? I know you said once they were low but things change over time? If it is the former you may want to look into the implantation clinic (I wondered if this was my problem so started to look into it a bit, phonixgirl had all the testing done if you want to ask her about it).
Hopefully it is just a slow riser of hcg and in a couple of days there will be an unmistakable line!