♥ Manufacturing Date: The number one reason is the age of the adhesive. At Wendylash, we are very picky about buying adhesive monthly so that your adhesive is always fresh when you purchase it. As soon as the adhesive is manufactured, it starts to lose its bonding strength. Ask when the adhesive was manufactured if you aren't sure. Every professional lash store should be able to tell you.
☆Pro Tip: Keep a log of when you opened your adhesive and replenish it monthly even if its not empty. Never buy more than you will need even if it is on sale!
♥Weather: More humidity in the air will always make adhesive dry faster, so consider switching to a slower adhesive during the summer.
♥ Changes: If neither of the above are right, then think about what changed in your environment or what products you are using. I always clean the lashes as much as possible to reduce any barrier between the adhesive and the natural lash. Using a gel cleanser or bubble cleanser makes a big difference. I also find primer helps to temporarily dry the lashes and open the hair follicles to make them bond better to adhesive.