How to work out which one it is
The pattern tells you almost everything, and it takes about thirty seconds of thinking rather than a consultation.
Bad in the first week, both eyes, evenly. Either the application or the first forty-eight hours. Did you shower, sweat, cry or steam within two days? If not, it is the set, and any decent studio will fix it free inside a week.
Fine for a week, then falling out quickly. Oil or cleaning. This is the most common pattern by far and it is entirely fixable at home.
Consistently worse on one side. Sleeping. Nothing else produces asymmetry.
Gradually worse over several sets. Worth asking whether the sets are getting heavier. A length that worked in March on healthy lashes can be too much in September on lashes that have been carrying it since.
What is not usually it
Humidity, which gets blamed constantly. It affects how fast adhesive cures at the moment of application, which is a studio problem solved by a hygrometer and choosing the right adhesive, not something that happens to you in July.
An allergy. Genuine cyanoacrylate allergy exists and is rare, and it presents as swollen, red, itchy lids within a day or two, not as poor retention. Itching without swelling is far more likely to be a dirty lash line.
Your own lashes being weak. Almost never. Everybody sheds two to five lashes an eye per day and always has.