If the Lord has promised to provide and is able, why doesn't He always meet our needs when we ask? Since He's completely faithful to His Word, the problem is obviously with us, not Him. In today's passage, His promise of provision has a condition--it's given to "those who walk uprightly" (v. 11). So if God isn't providing for our needs as we think He should, He may have a different plan for us. But we should also examine our lives for possible hindrances.
Sin. One reason our prayers may not be answered is because we've allowed sin in our lives and are not walking uprightly. If the Lord ignored disobedience and granted our requests, He'd be affirming a sinful lifestyle.
Laziness. Another possible explanation for unmet needs is that we haven't done our part. Although God is the ultimate source of all we have, He's given us the responsibility to work in order to provide basic necessities (2 Thess. 3:10-11). If you're an able-bodied person who's unwilling to work and wants something for nothing, the Lord is not going to reinforce your laziness.
Desires. Perhaps God hasn't provided as you expected because your "needs" are really desires. If He knows that what you want won't fulfill His plans for your life, He will withhold it in order to provide something better.
To avoid disappointment with God, understand that His actions and character always align. He won't reward rebellion or laziness, and His answers to prayer fit with His goal of conforming us to Christ's image. If He's withholding something you deem essential, He's working something even better for you.