It can be trying, but send letters and get documentation. If the creditor still refuses to budge and the collection account still shows up, an underwriter is inclined to be much more lenient about it if you can show that you have been attempting to correct the matter. They don't like it when people just ignore things and refuse to pay. Remember, it's all about what you can prove to the underwriter. Bankruptcies and tax liens also show up in the report.
Underwriters place varying importance on different types of credit.
They do not like seeing these. A late on one of these won't be as bad as a mortgage late, but it's still cause for concern. Then comes credit cards. Not that they brush off credit card lates, but you're not as bad off if you have a few lates here, but always make your house and car payments on time. Underwriters also view late payments differently depending on the type of loan you are applying for.