Vfp on FRER but bfn on others

Brittney • 10 year old boy and 2 year old girl 🖤
I took 2 FRER yesterday and both had faint pink lines. So I tried the wal mart cheapie and a blue dye and absolutely nothing.. bfn. This morning I took another frer and there's still a pink faint line showing up so I tried a blue dye and bfn! What is going on??