Candiace,
There is a limit to the number of individual field cells that can be formatted. That limit used to be something like 1024 but that may have changed. You say several tasks, but that doesn't tell us how many. Keep in mind that each individual field cell counts
as one format. If you format a group of cells at one time, the group is treated as one.
However, that being said, I recall reading something about this type of issue being a bug that was fixed with an update. So, first, make sure you have SP2 for Project 2010 installed. If that doesn't fix the problem right there, I suggest you update to the
latest cumulative update which is the December 2014 CU. You can download them at:
http://support.microsoft.com/kb/2687521
If neither of those fix your problem then I suggest you reconsider how much custom formatting you really need. You may be at the limit.
Hope this helps.
John