2 cats have bumps/scabs

[DanHumphreyIsA]

All three cats are deflead with a spot on (stronghold) regularly.
The 2 outdoor cats have got bumps on the skin. We had this with cat 1 (indoor) about 2 years ago, and were told it was likely because of fleas at the time. She hasn't had them since.

Is there a possibility stronghold has stopped working? They are due for new spot on in about a week, I noticed the first bump on one of them around a week ago.
Just now the same cat has atleast 3 bumps around his neck. (he has recently started wearing a collar, as moved to new area and he likes to wander, could it be a reaction?)
The other cat only has one scab on her belly, noticed yesterday. She doesn't wear a collar, doesn't like it and she never leaves the garden, anyway.
They don't really scratch that much, and I can't see any flea dirt.
I haven't got a flea comb to check that way though.

Calling the vets on Monday for both of them, but I am worried incase the spot on didnt work and they've had fleas for weeks now.
Indoor cat seems fine though and not scratching at all, so find it weird if they have fleas and haven't passed them onto her.

Could it be something other than fleas?

[Honkyzeke]

We had this years ago with our cats one much worse than the other but it turned out they had developed an allergy to fleas due to the flea treatment not working, the fleas bought them out in lumps and bumps which then turned into scabs. Hope you get it sorted.

[Weedsnseeds1]

Could the bumps be ticks? I have had a cat with flea allergy and she lost fur and ended up with a leathery bald rear end when bitten.

[harridan50]

Probably steroids from vets would resolve this good luck

[AllToadsLeadToHome]

Brush them with some diatomaceous earth (food grade). I used it to remove scabby bits from my cats, also treated the floor but you may not want the dust. It kills pretty much any insect, even the good ones though. Once the Frontline has had time to wear off you can use a better alternative.