General:
* Issues
* Challenges
* Difficulties
* Troubles
* Obstacles
* Dilemmas
* Predicaments
* Conundrums
* Quandaries
* Hardships
* Adversity
More specific:
* Technical: Bugs, glitches, errors, malfunctions
* Mathematical: Equations, puzzles, riddles
* Social: Conflicts, disagreements, disputes, grievances
* Personal: Worries, anxieties, burdens, stresses
* Financial: Debts, expenses, shortfalls, deficits
The best word to use depends on the context and the specific type of problem you are referring to.