Problems since updating to Yosemite

Hi - since updating to Yosemite my passwords for Outlook are not always 'remembered' every once in a while it asks me to put the password in again - I have been into Apple and the computer itself is working and they said it was probably an incompatibility problem with the software.  I presume there must be an update or easy way around this problem but I have not yet found it. Also since the update there is no ' full screen' option on Excel the two arrows in the top right hand corner - so I cannot view Excel files with multiple pages on my Mac Book pro - is the option of full screen somewhere else?? 

Thanks for any helpful info,

JacquieCC-1964