There are two schools of thought on this subject,
#1 fast controled opening with an exact placement will drop the deer in his tracks[I usually go this route its been years since I had to do any tracking on a gun deer]
#2 That a heavy bullet [270+] slowly opening will do the job[ and it will] and hopefully a nice exit hole to leave a blood trail and penatration from any angle through to the vitals.
I personally prefer the the 250 Gold Dot and either 110gr 777 or 120 or 130 RS depending on what the gun likes.
Now I am going into a highly contriversal area, the powder load, it has to be accurate but it also has to suit the bulletto much can open the bullet to fast which will reduce penatration and to little and it will also reduce penatration. The only bullets I use what I consider a mag load[over 130gr] with is the very heavy[ 325+] and the 200gr Shock Wave[ my extreme range bullet] it will hold together.
Since you did not mention the powder load but did mention that you use mag bullets I though maybe you are loading heavy; either way I know that I used 240 gr bullets for several years and a number of deer I recoverd all of them but had similar problems to what you are experiancing. You could go to one of the very good very expensive bullets in a barnes or nosler but personally I think that they are to expensive as I like to shoot a lot and use my hunting load so I know exactly what it will do. The Gold Dot 250 with the right amount of powder is a terrific bang flop killer but you need to put it in the right place , right be hind the fore leg,Bad angles and shoulder shots I avoid with the 250. The 300 gr XTP or Gold Dot will punch through shoulder [and spoil a bunch of good meat] and thats likly the best chance for a bang flop with them put squarly into the ribcage in one side and out the other they will bring down even very large game but you will usually have to trail it down. Lee