To my knowledge engines with plunger type oil pumps (Triumph, Ariel etc) Are not usually that prone to wet sumping. The first and easiest thing to check is the non return ball valve. Bottom left of the timing cover is a hex plug, bihind which is a 7/32" ball (#S70-7) and spring. https://www.draganfly.co.uk/triumph-shop/twin-cylinder/product/37748-ball-bearing-7-32/category_pathway-2
The ball might not be seating properly! After removal, squirt out the seat with say WD40 with the red straw attached and or an air line. Install a new ball, in case the original is misshappen, give the new ball a light sharp tap with a thin drift to create a good seat in the ally case.
Check the oil level in your primary case, as excesive oil in the crankcase will leak through the main beaings into your clutch housing .......and ultimately, onto the floor. Ron