Has he performed for you in the past?
How old is he?
Did he challenge the new ram?
Has he been smelling musky or getting a bit frustrated prior to tupping as any rams do at this time of year ( one of mine takes to head butting low hanging branches. )
His complete lack of lip curling / tongue waving, sniffing etc would suggest no / low sexual drive
Did you give him an mot?
Its a good idea to prep rams about 6 weeks before tupping. Trimming feet, checking their parts. Feeding them up or trimming them down. A good 4 in 1 ram bucket can certainly help with fertility for older rams.
It could be physical as in sore feet, bad back, respiratory problems etc but I would have at least expected some sniffing and lip rolling if his spirit was willing but his flesh was weak.