Can only get bfp with a certain test!Help!?

So ive been getting bfp on kmart american fare preg tests sense 10 dpo.Also i was due to get bf saturday and have not got it yet.Ive tried other tests and haven't been able to get a bfp..well actually on the day bf was due i got 2 pos on the Walgreen's brand test but the next day i took another one of the wg brand and it was neg..the last bfp i got was monday on the kmart brand and today i tested with another brand and it was neg.Any similar experiences