After you have been on an antidepressant such as Effexor fo several weeks, the medication has usually reached a steady level in your bloodstream and has results in a stable effect on your brain. Missing one dose will not significantly affect these things.
However, it is true that sometimes symptoms of depression or anxiety may worsen after a period of being well-controlled on a certain dose of medication. This may be due to stressors in your environment, changes in brain chemistry, interactions with other medications which reduce how effective your antidepressant dose is, and changes in your weight or body composition which change how the drug is distributed in your tissues.
Because it sounds as though Effexor was an effective medication for you for a few years and you are on a relatively low dose of it, you probably would benefit from having the dose increased to 112.5 to 150 mg initially and seeing how you respond. The maximum dose is 300 mg per day, so if you did not develop significant side effects and your anxiety persisted, the dose could be increased further. I would encourage you to discuss your anxiety with the physician that prescribed the medication for you so he or she can adjust your dosage or discuss other options with you.
Yes, it is common for symptoms of depression or anxiety to get worse even after they have been under good control on a certain dose of Effexor for some time. Because you are on a fairly low dose, I would first recommend going up to a higher dose before considering changing medications. This should be done only after discussing with the doctor who prescribed this medication for you.
Effexor is as effective as most other major antidepressants--there is likely no significant difference in efefctiveness between it and fluoxetine, sertraline or other common antidepressants.
I hope this has helped.