The contracts toolkit
When a contract goes wrong
Handling disputes
Set deadline for action
In simple cases where the other party has simply failed without any proper excuse to do what the contract clearly called for, and if telephone calls have failed to work, you should write a letter as soon as possible clearly pointing out the failure (preferably referring to or quoting the specific clause) and requesting that it be remedied by a certain stated time (you should allow a reasonable time for this to happen). The letter might also state that if the breach of contract is not made good by the deadline you will have no alternative but to refer the matter to a solicitor. In appropriate cases you might directly threaten to issue legal proceedings. But as a general rule it is never wise to threaten something if you are not prepared to carry it through.
Solicitor's letter
If the deadline expires, consider finding a solicitor and asking them to write a letter on your behalf. This is often effective, since it indicates that you have the will to pursue your claim, and need not be expensive. Always make sure the solicitor knows what you want, and make it clear that s/he should not take any further steps, or run up further costs, without your instruction.
The courts
If all else fails, you may need to take the dispute to court. However this should be seen as a last resort, since court proceedings can be expensive and lengthy and the outcome is often uncertain. Also the other contract party may have more money than you to risk losing in court. However there are many cases where you should not be put off taking matters to court. This might be where there is no real argument under the contract, the other party is simply refusing to do what he or she agreed, particularly where the other party is refusing to pay you money that is clearly owed, or your copyright is being blatantly ignored or infringed. Sometimes issuing a writ is the only way to ensure you are taken seriously.