You need to issue a CHECKPOINT to clear the log in SIMPLE recovery before shrinking.
If that doesn't fix it run the following in that database:
Check the Status column, look for the last row (Virtual Log file/VLF) where Status=2. That is as far back in the log file that you can shrink to (active portion of log). In order to shrink further you need to cycle the active log back to an earlier VLF. You can do that with:
CREATE TABLE testtable(i int not null default(0)
DECLARE @c int;
WHILE @c<1000 BEGIN
INSERT testtable DEFAULT VALUES;
DROP TABLE testtableOnce that completes you would repeat the process to shrink the file, including the CHECKPOINT.