Big Query SQL - i would like the last value in the results which is not null

Question

I have the following script which I am running in big query.

It is supposed to give me the last non zero value when ordered by date. and it works in most cases.

for some reason I am now getting some null values and I would also like these null values to show the last non zero value but can not get it to work.

The script is as follows, if anyone can help I would greatly appreciate it.

Select 
a.AccountNumber,
a.LoadDate,
a.TransactionDate,
a.PostDate,
a.VoidDate,
a.TransactionAmount,
a.TransactionCode,
a.NewCustomer
,b.InterestRate 

from (

SELECT 
  Accno AS AccountNumber, 
  Load_Date AS LoadDate, 
  trandate AS TransactionDate, 
  postdate AS PostDate, 
  voiddate AS VoidDate, 
  bookamt AS TransactionAmount, 
  txncode AS TransactionCode,
  b.CustomerCreateDate,
  case when b.CustomerCreateDate is null then null 
     when DATE_DIFF( cast (trandate as date), cast(CustomerCreateDate as date), day) <= 90 then 'New'
     ELSE 'Existing' end NewCustomer,
  DATE_DIFF( cast (trandate as date), cast(CustomerCreateDate as date), day) as Days
 FROM `utb-dw-prod.UTB1.vw_Txns`  a

left outer join (
    select  * from 
    (
      select *,
      ROW_NUMBER() OVER ( PARTITION BY AccountNumber ORDER BY CustomerCreateDate ASC ) AS rn
      from `utb-dw-prod.Reporting.DE003_Deposit_Customer` 
    )a
    where a.rn = 1 ) b on a.Accno = Cast( b.AccountNumber as Int64)

WHERE 

txflags <> 7168
and
txflags <> 3072
and
txflags <> 27648
and
txflags <> 31744
and
txflags <> 67140608
and
txflags <> 2048
and
(txflags < 10000000000 or (txflags = 1125899906867209 and bookamt < 0) or (txflags = 1125899906842633 and bookamt < 0))
and
trandate >="2021-01-01"
and
(voiddate = "1900-01-01" or voiddate < trandate)
and 
txflags & 16 != 16
and 
txflags & 2048 != 2048

) a

--left outer join `utb-dw-prod.BankCoreViews.vw_Deposit` b on a.AccountNumber  = cast( b.AccountNumber as int) and Cast (a.TransactionDate as Date) = b.snapshotdate

left outer join (
  select t.AccountNumber
        ,t.SnapshotDate
        ,case when t.interestRate = 0 then q1.lastValue else t.interestRate end  as interestRate,
  from `utb-dw-prod.BankCoreViews.vw_Deposit` t 
  left outer join
  (
    select 
      q.AccountNumber
      ,q.snapshotDate
      ,q.lastValue
    from
    (
      select t1.* , t2.interestRate lastValue  ,  RANK() over ( partition by t1.AccountNumber, t1.snapshotDate  order by  t2.snapshotDate desc    )  as RANK
      from `utb-dw-prod.BankCoreViews.vw_Deposit` t1
      left join `utb-dw-prod.BankCoreViews.vw_Deposit` t2 on cast(t1.AccountNumber as int) = cast(t2.AccountNumber as int)--and  t1.snapshotDate >= t2.snapshotDate   and  t1.interestRate=0 and t2.interestRate <>0
      where t2.interestRate is not null 
      and  t1.interestRate=0 and t2.interestRate <>0
      and  t1.snapshotDate >= t2.snapshotDate 
      
    ) as q  where q.rank = 1 
  ) as q1 on q1.AccountNumber = t.AccountNumber and q1.snapshotDate = t.snapshotDate

) as b on a.AccountNumber  = cast( b.AccountNumber as int) and Cast (a.TransactionDate as Date) = b.snapshotdate