Column Chart by Month Not Working

To understand the Availability calculation you will need some background:

The Shift_Data table is used to capture production output, which can either be captured by shift (so once at the end of the day; if two different types of products are made during the shift, there will be two records in this Shift_Data table for that shift) or by hour (so each hour is a new record in the shift data table).

The Availability measure uses the following calulation:

Availability = 1-
(SUM('Shift_Data'[Downtime])
/
(SUM('Shift_Data'[Available_Time]))
)

The "Downtime" shown above is a column from the Shift_Data table, while "Available_Time" is a calculated column shown below ("Type" indicates whether the data was captured hourly or by shift):

Available_Time= 
//this is the full hour/shift - planned downtime
if([Type]="Hour",
//Hourly available time:
60-[Planned Downtime],
//Shiftly available time:
if([Find_last_entry_for_same_Shift]="MAX",LOOKUPVALUE('Shift_Setup'[Total Shift time],'Shift_Setup'[ID],[Shift_Setup_ID])-[Planned Downtime]
))

There is another table called "Shift_Setup" which gives the total time for a shift when data is captured by shift, which is what is being referenced above. The "Planned downtime" above comes from the Shift_Data table. The "