Why is no juice left after cooking lamb or beef joint in a pan?
The way you describe it, you seem to be doing it quite right. 

So, let's have a look at where juces in a pan come from and where they might go:


Any juice in the pan comes either from the meat or from added liquids (like wine). 
You actually don't want your meat to loose it's water - that's what we call dry meat ;-) A small amount is fine, if you lose nothing, you a) obviously used good quality and b) did everything right with the searing.
If your meat exudes liquid during roasting but afterwards nothing is left, it has evaporated. That ist especially likely when using a convection oven. 


If you want gravy you could either


Add some liquid like broth, wine, port or even water (whatever goes with your type of meat and your personal preferences), spices (peppercorns, clove, bay leaf, allspice...) and a mirepoix when roasting your meat. Place the seared meat either in the liquid or on a rack above it to catch any drippings (which might not be much). Scraps from your meat / bones could go in, too. 
Make it separately (even beforehand!) from bones and veggies.