My Electrum seed does not work in another wallet
You did not copy it wrong. They are two different schemes that look so alike nobody told you they were different.
Electrum does not generate BIP-39 seeds. Its phrase carries a version number inside that says which derivation to follow. A wallet that only understands BIP-39 cannot read that number.
The two symptoms, and which one is worse
It rejects the phrase. The other wallet says the seed is invalid. Annoying, but honest: you found out immediately.
It accepts it and shows zero. This is the dangerous one. Some wallets accept any word list and derive anyway, because they treat the phrase as text and never check which scheme it came from. The result is a legitimate, functional, empty account — with no error message at all.
If that happened to you, the natural reaction is to assume the coins are gone. It almost never is.
How to tell which one you have
The real check is Electrum itself: if the phrase opens your wallet there, it is an Electrum seed, and the version number inside is what tells it which derivation to use (Electrum's seed phrase documentation).
One hint that is not proof: BIP-39 defines phrases of 12, 15, 18, 21 or 24 words and no other count. If yours is a different length it is certainly not BIP-39 — but having 12 words does not prove that it is.
What to do
- Restore it in Electrum, the program that understands its own format. If the balance appears, nothing was lost: the problem was the interpreter.
- If you want to move to another wallet, do not try to translate the phrase — there is no translation. Create a new wallet with the scheme you want and transfer the funds.
- Do not paste the phrase into websites offering to "convert" or "validate" seeds. No legitimate check needs your seed.
What this problem reveals
Your backup was complete and still was not enough, because a piece of information was missing that nobody told you to write down: which program created that wallet. Two lines next to the words — which wallet, and what year — would have saved this entire afternoon.
That is the general shape of the problem, and it has more faces: the address type, the passphrase, the derivation path. All of them share the uncomfortable property of never warning you.
The test that covers all of them is not "can I write my words again?" but: given only what you wrote down, can software that has never seen your wallet reproduce your first address? The ceremony runs exactly that check — and it works for an Electrum wallet too, because it operates on the extended public key, not on the phrase. It runs in your browser, never asks for the seed, and the page declares connect-src 'none'.