The allowance builds up in your account from the time your employer joins the scheme, assuming you are currently employed by the employer.

You may also join the employer AFTER the employer has joined the scheme and in that case your allowance would start from then, assuming you were unemployed before that.

If you were employed before that by a different employer you may already have some 'bankable' allowance you can bring forward.

It may also be that you work for 2 different employers and this also can affect the allowance you receive from your account