The new dog (2 y/o YLF) likes to switch. I can throw a bumber and toss two off to the sides while her back is to me and she'll get all three without a problem (in short grass), but if I throw more than one for her to mark she'll go to the last one thrown and take it to the next and drop it, pick the one there and so on till she gets to the first one thrown then retrieve it. After the first is delivered she'll go get the one she's sent to and bring it back without hesitation. What gives and how can I fix it? Check cord maybe?