Your phone does not respect Android standard and may need some manual configuration to prevent this from happening.
See the following site for solutions for most vendors.
Even if not obvious those settings can also generate missing images because their optimizer delete app cache while the application is running. (Specially true for Huawei)
Note: Symfonium now offer an advanced setting (Persistent image cache - [Wiki] Settings - Advanced) to improve the thumbnail issues in most of the cases.