LEAD/LAG SQL Server 2012/Gaps and Islands

I'm having a few issues with LEAD/LAG. For each row within a set of IDs I'm wanting to get the previous/next source where isAQI = 1. Desired output is as follows in prevAQI and nextAQI columns.

I've tried the same approach as Lag() with conditon in sql server, but with no luck. Any help would be much appreciated!

Sample data as follows:

DECLARE @a TABLE ( id int, timest datetime, source char(2),
                   isAQI int, prevAQI char(2), nextAQI char(2))
INSERT @a VALUES
(6694   ,'2015-06-11 08:55:06.000'  ,'I'    ,1, NULL, 'A'), 
(6694   ,'2015-06-11 09:00:00.000'  ,'A'    ,1, 'I', 'I'),
(6694   ,'2015-06-11 09:11:49.000'  ,'C'    ,NULL, 'A', 'I'),
(6694   ,'2015-06-11 09:29:06.000'  ,'O'    ,NULL, 'A', 'I'),
(6694   ,'2015-06-11 09:29:06.000'  ,'DT'   ,NULL, 'A', 'I'),
(6694   ,'2015-06-11 09:34:11.000'  ,'DT'   ,NULL, 'A', 'I'),
(6694   ,'2015-06-11 09:34:11.000'  ,'O'    ,NULL, 'A', 'I'),
(6694   ,'2015-06-11 10:06:27.000'  ,'I'    ,1, 'A', 'I'),
(6694   ,'2015-06-11 11:25:09.000'  ,'DT'   ,NULL, 'I', 'I'),
(6694   ,'2015-06-11 18:25:24.000'  ,'C'    ,NULL, 'I', 'I'),
(6694   ,'2015-06-12 17:57:16.000'  ,'I'    ,1, 'I', NULL);

SELECT *
FROM @a

See if the following query works for you:

WITH C AS
(
  SELECT *,
    MAX(goodval) OVER(PARTITION BY id
                      ORDER BY timest
                      ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) AS prv,
    MIN(goodval) OVER(PARTITION BY id
                      ORDER BY timest
                      ROWS BETWEEN 1 FOLLOWING AND UNBOUNDED FOLLOWING) AS nxt
  FROM @a
    CROSS APPLY ( VALUES( CONVERT(VARCHAR(23), timest, 121) 
                          + CASE WHEN isAQI = 1 THEN source END ) ) AS A(goodval)
)
SELECT id, timest, source,
CASE WHEN prv IS NOT NULL THEN SUBSTRING(prv, 24, 2) END AS prevAQI,
CASE WHEN nxt IS NOT NULL THEN SUBSTRING(nxt, 24, 2) END AS nextAQI
FROM C
ORDER BY id, timest;