IEF - problems with detaching gel

[STEFFF]

Hi!
I already posted my question in the molecular biology techniques forum, but this forum might be more appropriate.
I have a problem with gels cast using disposable plastic cassettes.
I've cast IEF gels (6 % containing 8M Urea/6 % Ampholytes 4-6, polymerized with 0.11 % TEMED and 0.03 % APS) and ran an IEF separation (1h at 150 V, 1h at 250 V. 15h at 350 V, 1h at 500V). After the run, the gel was detached from the cassette (see picture), resulting in no current.
I don't know, when the detachment happened, since I ran the IEF separation over night, but I stained the gel the last time and saw, that the proteins were stuck in the wells. So I think, that the incident happened early after starting the run.
I ran the separation under the same conditions before, and had great separations (several times, not just once). The problems started some weeks ago, I am using the same batch of casting cassettes as well as the same reagents as I did before. Does anybody have any suggestions?