A common misconception about radio antennas is that bigger size and higher gain are automatically better. In fact, in many cases a smaller and lower gain antenna could have better performance.
Understand your objective in changing antennas. In the WiFi arena, individuals often seek out a different antenna to increase download speed. Antennas do not increase speed. They impact the direction of the signal and beamwidth of the receive and transmit path, which can cause signals to come in weaker or stronger.
If the signal increases, additional communications distance is possible. An increase in signal CAN lead to an increase in speed if the reason for slow speed is weak signal. If the reason for slow speed is a congested network, an increase in signal may not mean an increase in speed. If the reason is obstacles, the signal may not increase at all.
This does not mean your antenna is broken, it means the issue you are experiencing was not fixed by the advantage of the new antenna. 
Imagine our friend Buster is frustrated that his commute to work on the highway takes one hour every day, and it's just a ten-mile drive. Without any further data, we might suggest Buster purchases a faster vehicle. Logically if his car can go faster, he should get there sooner. Buster calls us the next day dumbfounded because even in his new car, the commute took exactly the same amount of time.
But then we find out the reason Buster's commute took long was because traffic is at a standstill during morning rush hour when he goes to work. No matter how fast his car can technically go, it's travel is impeded by the presence of so many other vehicles.
While this analogy seems absurd- Buster obviously would have seen the other cars and known that traffic was his problem and a faster card would not help- this same exact thing can happen with antennas, but because we can't see what is impeding our performance, we don't know what solutions to try. We often get outreach expressing a new antenna did not improve speed or range. Learning about antennas science can help us understand what type of antenna could be best for our needs. Below is a popular introduction to antenna basics from Hackaday. But if you don't want to know that much, beneath the video are some simple basics that could help.
Basic guidelines to picking the right antenna:
1. Determine if you need directional or omnidirectional
A directional antenna concentrates gain in one direction. An omni will receive in all directions horizontally. A directional antenna is a good choice for fixed locations when you know the signal origin. An omni is better for mobile use where signals are in different directions.
2. Determine the right gain for you need
A higher gain such as 8 or 10 dbi may have a very narrow vertical path. This is good to cover long distance over open area, but for residential environments, a medium gain such as 5-6 dBi could be better.
3. Evaluate cable and pigtail choices- anything in between your radio and antenna will degrade performance. In most cases, such as using a short coax cable or barrel connector, the degredation will be minor and hardly noticable. But with thin-shielded and longer runs of coax, you may experience significant loss. Learn more about coax shielding
If you have trees or other buildings nearby and have the capability to do so, mounting an antenna on a pole or tower can help boost distance and get around obstacles.
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