You could have a partially obstructed tank filter or a weak lift pump from struggling with the alternative fuel earlier. Or even a gunked up lift pump.
Even though you have switched back to diesel make sure enough fuel is reaching the pump with enough pressure to make fuel flow from the return line off the injection pump.
No flow from a couple of elements could indicate not enough fuel is getting to the pump to keep the elements loaded properly. The only fly in the ointment so to speak is usually it is the first two elements that dry up first with inadaquate fuel. If there is no fuel coming out the return line when running try feeding the lift pump with a bottle of fuel direct.
Being the last two elements might indicate air in the fuel though as a possibility as well. I have always thought since my intellect is somewhat limited the burning of alternative fuels does have a hiden price.
You have to be willing to take the additional time to deal with troubles that may not occur running straight diesel for the life of the car. Of course your pump might be bad but I would certainly check very carefully prior to going there. What one thinks is really irrelevant until other things have been properly eliminated.
The clue here is it is the two non producing or weak producing elements are side by side at one end of the pump. If it were any two in the centre area I would still conduct the same tests. Just the end result might turn out to be more serious.
If the spare pump has just been laying around I would not trust it. In my experience getting a used pump that has sat full of diesel is still a risk but a better one. The only way I will trust a used average pump in any storage of more than a month or so is if it is held full of fuel. You could luck out though. Just my two cents worth.