Typically the catheter is inserted 1-1.5cm into the vein and should be positioned just outside of the heart, for consistency measure this length on the catheter and mark it, then advance into the vein until you reach the mark.

You may be advancing too far, but you might also be using too high dose of anaesthesis if it is possible use gaseous agents such as isoflurane (and give buprenorphine s.c during the surgery for post-op analgesia) which rats recover from more quickly. or there may be clots if you haven't heparinized the vein during surgery.

Hope this helps. 

http://www.ableweb.org/volumes/vol-17/10-gray.pdf

http://www.sciencedirect.com/science/article/pii/S1385299X0200185X

One option is to put the catheter on the other end of the rat from the brain, e.g., threaded in the femoral vein up to the vena cava.  The only problem here is making sure the catheter is long enough that it is in the vena cava. It should be easy to find a description of this common procedure.  Use a trocar to thread the distal end of the catheter to the same exit wound as you would use for the jugular.  This procedure in no way compromises the blood flow to the brain and puts the infusate just upstream from the right atrium.  I endorse the suggestion to switch to isoflurane and buprenorphine, especially if you are having to supplement the k/x during surgery when the surgery goes longer than expected.  Any unexpected mortality such as this should be reported to your attending veterinarian -- the vet staff has experience with this procedure and they will help you eliminate all of the mortality.  Any of the suggestions I have made would require a significant change to your IACUC protocol before you implement them.

It depends on the age (size) of the rats. I routinely perform this surgery in rats weighing anything from 250g up to 500g. I use the cannula for drug administration/ blood sampling in freely moving conscious rats 3-5 days after surgery. I always insert the cannula 3cm into the jugular vein. I measure and mark the cannulae with a marker pen before autoclaving them. To know where to make the incision (as obviously inserting it 3cm will depend on what part of the vein you access) with the rat lying on its back imagine a line from the centre of the lower jaw to the the animals right forearm and another line from the animals right ear to its left forearm (or vice versa depending on what side your cannulating)-make your incision where the lines would intersect. I've done this thousands of times and my rats never die. I think you're problem is most likely the anaesthetic. I use isofluorane